RC7620 2G voice calls do not return CONNECT but

I have noticed that the RC7620-1 (R8 and R9 Firmware) does not return CONNECT when dialling outgoing voice calls but there seems to be a workaround:

RC7620 with EE SIM forced to 2G network, diallng out to my O2 phone on 4G network
ATD; returns OK
When I answer my phone the RC7620 does not respond with CONNECT but the call is active as I hear audio.

If I then press a DTMF key on my phone, the RC7620 responds with CONNECT on receipt of the tone.
Alternatively if I send an “AT” command from the RC7620 it responds with CONNECT. Therefore my workaround is to dial, wait for OK and then send continuous AT commands to stimulate a CONNECT.
Similar it does not detect remote-end hanging up unless I do the above too.

If RC7620-1 is on 4G then the CONNECT response is received as soon as the call is answered, no workaround is required.

Is this is bug in the firmware?

Thanks

seems same issue as this one, you probably need to implement the workaround at this stage