Thanks for the reply. I'll try line switching. In the meantime, here is the log from Monitor, after the last digit in the phone number is dialled:
8686mS CMTARGET: LOOKUP CALL ROUTE:4 type=100 called_party=8303994 sub= calling=904 in=0 complete=0
8687mS CMTARGET: ADD TARGET:4 number=8303994 type=100 depth=1 nobar=1 setorig=1
8687mS CMTARGET: SYS SC:4 8303994 0 sc=type=Dial code=?, num=.
8688mS CMTARGET: LCR NOT MATCHED:4 (cpn=8303994)
8688mS CMTARGET: DIAL LINE:4 GROUP=0 SUCCESS=1
8689mS CMTARGET: OVERLAP LOOKUP CALL ROUTE:4 returned 1
8689mS CD: CALL: 0.1006.0 State=0 Cut=1 Music=0.0 Aend="Fileroom(904)" (5.3) Bend="Line 3" [Line 3] (0.0) CalledNum=8303994 () CallingNum=904 () Internal=0 Time=7236 Astate=7
8690mS CMExtnTx: v=904, p1=1
CMSetupAck
Line: type=DigitalExtn 5 Call: lid=0 id=1006 in=0
Called[8303994] Type=Default (100)
BChan: slot=5 chan=3
Display [8303994]
Timed: 06/03/07 13:52
8692mS CD: CALL: 0.1006.0 State=0 Cut=1 Music=0.0 Aend="Fileroom(904)" (5.3) Bend="Line 3" [Line 3] (0.0) CalledNum=8303994 () CallingNum=904 () Internal=0 Time=7239 Astate=8
13914mS PRN: CALL: 0.1006.0 Deleted leaving 0 CMCall objects
13914mS CALL:2007/03/0613:52,00:00:00,000,904,O,8303994,8303994,Fileroom,,,1,,""
13915mS CD: CALL: 0.1006.0 State=0 Cut=0 Music=0.1 Aend="Fileroom(904)" (5.3) Bend="Line 3" [Line 3] (0.0) CalledNum=8303994 () CallingNum=904 () Internal=0 Time=12462 Astate=0
13916mS CMExtnTx: v=904, p1=1
CMInbandTone
Line: type=DigitalExtn 5 Call: lid=0 id=1006 in=0
UUI type=User2User [TB1\r\n] [0x54 0x42 0x31 0x0d 0x0a ]
Cause=17, User busy
Timed: 06/03/07 13:52
13918mS CD: CALL: 0.1006.0 Deleted