If you are using Google Voice service, then this issue is clearly Google Voice service doing. If you are not using Google Voice service but other VoIP Services, reach out to them to see if all calls received and not able to Transfer. Inquire for more info such as calls logs from your VoIP Service team. Or, explain in further details in what you are using to get dropped when transferring a call from 1 OBi phone to another OBi phone. Without further details, we are unable to help.
Good luck!