For the purposes of testing, if you connect the phone direct to the VM hub (using the adapter) and leave the trueCall out of the equation, does caller ID show up on the phone handset reliably and does the phone behave normally with regard to ringing etc., minus the trueCall unit?
It would probably also be worth placing a call to trueCall support for further advice. There is a difference in the connections via the hub adapter compared with a telephone master socket and how ringing is presented to the trueCall device.
Past topics on here regarding trueCall issues have usually been related to caller ID on VM lines. In those past cases, trueCall have been able to remotely dial into the trueCall unit and tweak the configuration to suit the VM line. This may need something similar but trueCall would be best placed to advise on this (assuming there is no fault with the VM connection itself, which you should be able to test using just the phone itself and no trueCall device).