Further thoughts would be:
Is the VM landline phone connection provided via the back of the hub (a 21CV connection)?
If so, are you connecting the trueCall direct into the hub via the adapter (not via any phone extension sockets around the home)?
Is the caller ID service working normally when just the phone is plugged in?
When you ring into your VM landline number from your mobile (with both phone and trueCall connected) what messages does the trueCall announce to indicate how it is processing the call? (The trueCall response will depend if you have added your mobile number to the 'star' list or have left it as 'unknown'. You might have to try both)
Do you have access to the trueCall 'Internet Control Panel' (their online subscription service where you can see all calls coming in/out of the device and how they are being processed)? That might also give some clues as to what is happening? (If you are not using the 'Internet Control Panel' service, that should have no adverse effect on the operation of the unit.)
If none of the above sheds any light, you may need to speak to trueCall direct. I believe they are able to 'dial-in' to the device, if required, for troubleshooting.