Check the firmware on your Yealink SIP-T46G phones.
As you know: they are discontinued now.
Only the SIP-T46S is still alive (in production).
The last and only firmware you should have on a SIP-T46G is:
T46-28.83.0.120.rom
If the phone is not using this FW: upgrade it because this is the best one of the breed.
TIP
Register your SIP-T46G with the 71 and 72 set (to another) FPBX system. Test the phone on a different system with working parking to see if it is the phone and/or the PBX … (as a starting point).