Here is what I pulled from Sysmon:
16:47:10 2660420301mS VMAIL: VMMESSAGE Transfer request [$1539@1539]
16:47:10 2660420301mS CMExtnRx: v=RAS, p1=0
CMTransfer
Line: type=NoLine 0 Call: lid=0 id=-1 in=0
Called[$1539] Type=Default (100) Reason=CMDRdirect SndComp Calling[1539] Type=Unknown Plan=Default
IE CMIECallingPartyName (110)(Type=CMNameDefault) name=1539
16:47:10 2660420301mS CMCallEvt: 0a02651400056a3d 0.354877.0 39914 RAS.0: Transfer CMCauseTransfer
16:47:10 2660420301mS CD: CALL: 2.4725.1 BState=Connected Cut=3 Music=0.0 Aend="Line 2" (252.2) Bend="ConfMenu(ConfMenu)" [VoiceMail] (0.0) CalledNum=ConfMenu () CallingNum=97737996241 () Internal=0 Time=6558 AState=Connected
16:47:10 2660420301mS CMTARGET: 0a02651e00013ae1 2.4725.1 39914 H323TrunkEP: ADD TARGET (N): number=$1539 type=100 depth=1 nobar=1 setorig=1 ses=0
16:47:10 2660420301mS CMTARGET: 0a02651e00013ae1 2.4725.1 39914 H323TrunkEP: SYS SC: $1539 5 1539 sc=type=ConferenceRendezvous code=$XXXX, num=. callinfop->sending_complete=1 secondary_dialtone=
16:47:10 2660420301mS CMTARGET: Conference id to be targeted is $1539
16:47:10 2660420301mS CMMap: PCG Conference create g C[984] (1.506)
16:47:10 2660420301mS CMCallEvt: 0000000000000000 0.354878.0 -1 BaseEP: NEW CMEndpoint f6bbf0f8 TOTAL NOW=9 CALL_LIST=4
16:47:10 2660420301mS CMTARGET: 0a02651e00013ae1 2.4725.1 39914 H323TrunkEP: PrepareTransferTargets Found 1 target
16:47:10 2660420301mS CMTARGET: 0a02651e00013ae1 2.4725.1 39914 H323TrunkEP: GetNoAnswerTimer:20
16:47:10 2660420301mS CMCallEvt: 0000000000000000 0.354879.0 -1 BaseEP: NEW CMEndpoint f6bddf48 TOTAL NOW=10 CALL_LIST=4
16:47:10 2660420301mS CMExtnEvt: RAS: CALL LOST (CMCauseTransfer)
16:47:10 2660420301mS CMCallEvt: 0a02651400056a3d 0.354877.0 -1 RAS.0: StateChange: END=X CMCSConnected->CMCSCompleted
16:47:10 2660420301mS CMExtnEvt: v=1011 State, new=PortRecoverDelay old=Connected,0,0,RAS
16:47:10 2660420302mS CMExtnTx: v=RAS, p1=0
CMReleaseComp
Line: type=RAS 1 Call: lid=0 id=1011 in=0
Called[ConfMenu] Type=Voicemail (102) Reason=CMDRX_Attendant Calling[97737996241] Type=Unknown Plan=Unknown Pres=Allowed (0)
Cause=126, Transfer(IPO)
16:47:10 2660420302mS H323Evt: SESS 26e9: RTP(END): 127.0.0.1/37264 10.2.101.20/51450 CODEC=Ulaw64K(5) PKTSZ=160 RFC2833=off AGE=6558 SENT=0 RECV=0 RTdelay=0 jitter=0 loss=0 remotejitter=0 remoteloss=0
16:47:10 2660420302mS VMAIL: SESS 2743: CMD=130 Mailbox=ConfMenu Access=4 2
16:47:10 2660420302mS VMAIL: VMAIL Open [VMMESSAGE_UTF8CLOSE] [UTF8] info :-
16:47:10 2660420302mS VMAIL: Flags :- 65 6e 75 00 00 00 9b ea
16:47:10 2660420302mS VMAIL: 00 00 7e 66 01 80 4c 4c
16:47:10 2660420302mS VMAIL: Locale: enu
16:47:10 2660420302mS VMAIL: Calling Party number: 97737996241 Display String: 97737996241>ConfMenu
16:47:10 2660420302mS VMAIL: Greeting Modifier:
16:47:10 2660420302mS VMAIL: Targeted Party number: ConfMenu Called Party number:
16:47:10 2660420302mS VMAIL: Calling Party name: Called Party name:
16:47:10 2660420302mS VMAIL: DID number 5088851500 QUEUE: CallAge=0 QueueAge=0 QPos=0 QEta=0 AnnouncmentSync=0
16:47:10 2660420302mS VMAIL: Recording Originator: IP addr. of Recording Originator: 0.0.0.0
16:47:10 2660420302mS VMAIL: Targeted Party Name:
16:47:10 2660420302mS VMAIL: Recording Type: Auto Recording Incoming HuntGroup
16:47:10 2660420302mS VMAIL: Answering Party Number: Answering Party Name:
16:47:10 2660420302mS VMAIL: Targeted Party Offset: 4c
16:47:10 2660420302mS VMAIL: Compression Format: input[00] output[00]
16:47:10 2660420302mS VMAIL: Validated Recording: No
16:47:10 2660420302mS VMAIL: UTC Date: 2020/01/13
16:47:10 2660420302mS VMAIL: UTC Time: 21:47:03
16:47:10 2660420302mS VMAIL: User-defined-data 00-00000000-00000000
16:47:10 2660420302mS VMAIL: SESS 2743: END Transmit=0 Discards=0 Recv=0 SeqErrs=0 Empty=0 Purges=1
16:47:10 2660420303mS CMExtnEvt: RAS: CMExtnHandler::SetCurrent( id: 354877->0 )
16:47:10 2660420303mS CMCallEvt: 0a02651400056a3d 0.354877.0 -1 RAS.-1: StateChange: END=X CMCSCompleted->CMCSDelete
16:47:10 2660420303mS CMExtnEvt: v=1011 State, new=Idle old=PortRecoverDelay,0,0,RAS
16:47:10 2660420303mS CMExtnTx: v=RAS, p1=0
CMReleaseComp
Line: type=RAS 1 Call: lid=0 id=1011 in=0
Cause=126, Transfer(IPO)
16:47:10 2660420303mS CMCallEvt: 0000000000000000 0.354879.0 39914 TargetingEP: StateChange: END=B CMCSIdle->CMCSOffering
16:47:10 2660420303mS CMCallEvt: 0a02651400056a3e 0.354878.0 39914 Conf $1539.-1: StateChange: END=T CMCSIdle->CMCSOffering
16:47:10 2660420303mS CMCallEvt: 0a02651400056a3e 0.354878.0 39914 Conf $1539.-1: StateChange: END=T CMCSOffering->CMCSConnReq
16:47:10 2660420303mS CMCallEvt: 0000000000000000 0.354879.0 39914 TargetingEP: RequestEnd 0a02651400056a3e 0.354878.0 39914 Conf $1539.-1
16:47:10 2660420303mS CMTARGET: 0a02651e00013ae1 2.4725.1 39914 H323TrunkEP: CancelTimer CMTCNoAnswerTimeout
16:47:10 2660420303mS CMCallEvt: 0a02651400056a3e 0.354878.0 39914 Conf $1539.-1: StateChange: END=B CMCSConnReq->CMCSConnected
16:47:10 2660420303mS CMTARGET: 0a02651e00013ae1 2.4725.1 39914 H323TrunkEP: ~CMTargetHandler f6b98bd0 ep efe09810
16:47:10 2660420303mS CMLineTx: v=2
CMFacility
Line: type=IPLine 2 Call: lid=2 id=4725 in=1
IE CMIERespondingPartyName (228)(Type=CMNameInternalConference) name=Conf $1539
IE CMIERespondingPartyNumber (230)(P:100 S:100 T:116 N:100 R:4) number=$1539
IE CMIEDeviceDetail (231) 0a02651400056a3e LOCALE=enu HW=11 VER=11 class=CMDeviceConference type=0 number=8388823 channel=1 features=0x1 rx_gain=32 tx_gain=32
ep_callid=354878 ipaddr=10.2.101.20 apps=10 loc=0 em_a_loc=0 em_d_loc=0 features2=0x0 is_spcall=0 ignores_dtmf=0 avgsid=
16:47:10 2660420304mS CMCallEvt: ConfMedia $1539: member=0a02651400056a3e 0.354878.0 39914 Conf $1539.0 mute=0 bothway=1 music=1 rxgain=32
16:47:10 2660420304mS CMLineTx: v=2
CMFacility
Line: type=IPLine 2 Call: lid=2 id=4725 in=1
IE CMIERespondingPartyName (228)(Type=CMNameInternalConference) name=Conf $1539
IE CMIERespondingPartyNumber (230)(P:100 S:100 T:116 N:100 R:4) number=$1539
IE CMIEDeviceDetail (231) 0a02651400056a3e LOCALE=enu HW=11 VER=11 class=CMDeviceConference type=0 number=8388823 channel=1 features=0x1 rx_gain=32 tx_gain=32
ep_callid=354878 ipaddr=10.2.101.20 apps=10 loc=0 em_a_loc=0 em_d_loc=0 features2=0x0 is_spcall=0 ignores_dtmf=0 avgsid=
16:47:10 2660420304mS CMCallEvt: ConfMedia $1539: CheckActive=1 marks=00080010
16:47:10 2660420304mS CMCallEvt: ConfMedia $1539: CheckActive=1 marks=00080010
16:47:10 2660420304mS CMCallEvt: 0000000000000000 0.354879.0 -1 TargetingEP: StateChange: END=X CMCSOffering->CMCSDelete
16:47:10 2660420304mS CMCallEvt: 0000000000000000 0.354879.0 -1 BaseEP: DELETE CMEndpoint f6bddf48 TOTAL NOW=9 CALL_LIST=4
16:47:10 2660420304mS CMCallEvt: ConfMedia $1539: CheckActive=1 marks=00080010
16:47:10 2660420304mS CMCallEvt: ConfMedia $1539: member=0a02651400056a3e 0.354878.0 39914 Conf $1539.0 mute=0 bothway=1 music=1 rxgain=32
16:47:10 2660420304mS CMCallEvt: ConfMedia $1539: CheckActive=1 marks=00080010
16:47:10 2660420304mS CMLineTx: v=2
CMFacility
Line: type=IPLine 2 Call: lid=2 id=4725 in=1
IE CMIERespondingPartyName (228)(Type=CMNameInternalConference) name=Conf $1539
IE CMIERespondingPartyNumber (230)(P:100 S:100 T:116 N:100 R:4) number=$1539
IE CMIEDeviceDetail (231) 0a02651400056a3e LOCALE=enu HW=11 VER=11 class=CMDeviceConference type=16 number=8388823 channel=1 features=0x1 rx_gain=32 tx_gain=32
ep_callid=354878 ipaddr=10.2.101.20 apps=10 loc=0 em_a_loc=0 em_d_loc=0 features2=0x0 is_spcall=0 ignores_dtmf=0 avgsid=
IE CMIECause (255) Cause = 126
Cause=126, Transfer(IPO)
16:47:10 2660420304mS CMLineTx: v=2
CMFacility
Line: type=IPLine 2 Call: lid=2 id=4725 in=1
IE CMIESupplementaryService (3)
Interpretation APDU
rejectAnyUnrecognisedInvokePdu
CallTransferComplete.Invoke.CodePageUtf8String
invokeId 15612
redirection name 'Conf $1539' redirection number '' secondary end answered
IE CMIERespondingPartyName (228)(Type=CMNameInternalConference) name=Conf $1539
IE CMIERespondingPartyNumber (230)(P:100 S:100 T:116 N:100 R:4) number=$1539
IE CMIEDeviceDetail (231) 0a02651400056a3e LOCALE=enu HW=11 VER=11 class=CMDeviceConference type=16 number=8388823 channel=1 features=0x1 rx_gain=32 tx_gain=32
ep_callid=354878 ipaddr=10.2.101.20 apps=10 loc=0 em_a_loc=0 em_d_loc=0 features2=0x0 is_spcall=0 ignores_dtmf=0 avgsid=
16:47:10 2660420304mS CMCallEvt: 0a02651400056a3d 0.354877.0 -1 BaseEP: DELETE CMEndpoint f0143210 TOTAL NOW=8 CALL_LIST=4
16:47:10 2660420305mS CMMap: PCG::UnmapBChan pcp[3]b1r1 cp_b f437c304 other_cp_b f3c05b70
16:47:10 2660420305mS JadeMapper: Sending Map B Chan: 0x1 0x1f9 0x1 0x1 0 2
16:47:10 2660420305mS CMMap: PCG::UnmapBChan pcp[635]b0r1 cp_b f014ab5c other_cp_b 0
16:47:10 2660420305mS CMMap: PCG::UnmapBChan pcp[635]b0r1 cp_b f014ab5c other_cp_b f437c304
16:47:10 2660420305mS JadeMapper: Sending Map B Chan: 0x1 0x1 0x1 0x1f6 0 2
16:47:10 2660420305mS CMMap: PCG::UnmapBChan pcp[3]b1r1 cp_b f437c304 other_cp_b 0
16:47:10 2660420306mS JadeRemoteMapper: PlayTone: Attempting to play wav file: /opt/ipoffice/tones/enu/../holdmusic1.wav
16:47:10 2660420306mS JadeRemoteMapper: PlayTone: Attempting to play wav file: /opt/ipoffice/tones/enu/../holdmusic1.wav
16:47:10 2660420308mS JadeMapper: Deallocated media RTP port: 51450 (inUsePorts: 7, activeChannels: 7, freeChannels: 2041, activeChannelFreed: 1)
16:47:10 2660420308mS JadeMapper: Media RTP port: 51450 is already freed (inUsePorts: 7, activeChannels: 7, freeChannels: 2041)
16:47:10 2660420308mS CD: CALL: 2.4725.1 BState=Connected Cut=3 Music=1.0 Aend="Line 2" (252.2) Bend="" [Conf 1] (240.1) CalledNum=$1539 () CallingNum=1539 (1539) Internal=0 Time=6565 AState=Connected
16:47:10 2660420308mS PRN: Config Write Wake Up
16:47:10 2660420804mS CMMap: PCG::MapBChan g C[984] cp_b f6b7e568 other_cp_b f3c009e0 type CGTypeDTMF
16:47:10 2660420804mS JadeMapper: Sending Map B Chan: 0x1 0x1f9 0x1 0x1fa 1 2
16:47:10 2660420804mS CMMap: DTMF:

layTones g T[985] Tones 'E'
16:47:10 2660420804mS CMMap: Started playing tones, C[984] (from g null)
16:47:10 2660420805mS CMMap: C[984] SetOperGroup1 from g null to g null not executed, playing tones in progress E
I'm not really sure what to make of that information. It looks like in the first line that the conference is being connected to using its designated extension (1539) so I would think the call shouldn't be put on hold for the caller.