I've been pulling my hair out on this together with max and can't for my life figure out where the problem lies.
To give you all mor info, here's what's happening:
Receiving calls on DECT phones works just fine.
Pre-dialing (dial digits and then the off-hook button) works nicely as well.
Post-dialing (off-hook first and then dialing the digits) or pressing the flash key to get the second line will give a busy-tone after pressing the second digit (unless we dial a 0 first, which is for an external line).
It's as if we had a configuration setting somewhere telling the system that all numbers consist of one digit only. Here's a trace showing Extension 123 calling 158, then attempting to transfer to 159 (I assure you, the # key was never pressed):
[tt] Calling party station 123 cid 0x244d
Calling Number & Name 123 Test
dial 158
ring station 158 cid 0x244d
dial 158
seize trunk-group 19 member 11 cid 0x244e
Calling Number & Name NO-CPNumber NO-CPName
Setup digits 158
Calling Number & Name 123 NO-CPName
Proceed trunk-group 19 member 11 cid 0x244e
Alert trunk-group 19 member 11 cid 0x244e
G711A ss

ff ps:20
rgn:2 [10.20.10.24]:16402
rgn:1 [10.10.9.12]:2078
xoip options: fax

T modem

ff tty

T uid:0x50083
xoip ip: [10.10.9.12]:2078
active station 158 cid 0x244d
G711A ss

ff ps:20
rgn:2 [10.20.10.24]:16402
rgn:1 [10.10.11.78]:2322
G711A ss

ff ps:20
rgn:1 [10.10.11.78]:2322
rgn:2 [10.20.10.24]:16402
conf/tran hold station 158 cid 0x244d
active station 158 cid 0x2454
G711A ss

ff ps:20
rgn:2 [10.20.10.24]:16402
rgn:2 [10.20.10.12]:2070
xoip options: fax

T modem

ff tty

T uid:0x50083
xoip ip: [10.20.10.12]:2070
dial 1#
denial station 158 cid 0x2454
idle station 158 cid 0x2454[/tt]
Similarly, attempting to conference in/transfer to an external number:
[tt]... same initial call setup
conf/tran hold station 158 cid 0x244d
active station 158 cid 0x245f
G711A ss

ff ps:20
rgn:2 [10.20.10.24]:16402
rgn:2 [10.20.10.12]:2070
xoip options: fax

T modem

ff tty

T uid:0x50083
xoip ip: [10.20.10.12]:2070
dial 01# route:ARS
denial event 1751: No AAR/ARS route pat/pref D1=0x8c61 D2=0xc1c0019
dial 01# route:ARS
denial station 158 cid 0x245f
idle station 158 cid 0x245f
[/tt]
Any hints would be greatly appreciated.