Well, with VoLTE currently being implemented in networks and phones, that SIP stack used to realize VoLTE on the phone can pretty much be considered firmware to

Well, with VoLTE currently being implemented in networks and phones, that SIP stack used to realize VoLTE on the phone can pretty much be considered firmware too. However, you might fiddle around with various other SIP clients and find one that works for you. The other hundreds of millions of mobile phone users are not going to do that - so the problem will persist.