AlasdairL,
Apparently you do not have MoH or it should be providing that when the call is put on hold prior to the transfer to the phantom port. So, most likely it was designed to not use MoH nor set up in the Trunk Group to Use Next Device's Audio Source, which would allow you to change those characteristics on a per device basis.
Look at your CO Trunk Group for your ISDN line and see what each of the Music on Hold [MoH] setting are set for.
System > Devices and Feature Codes > CO Trunk Groups > {trunk group}
Audio for Calls Camped onto this Device
Music-On-Hold
Audio on Transfer to Ring
Audio on Transfer to Hold
Audio on Hold for Transfer Announcement
Thanks,
TE