Hi forum members, I am struggle with phone few days and get to register phone on 3cx (15.5 SP4). Found many topics on 3cx forum where guys have a several problems like I do. Problem is that every second call is passing. With wireshark it is clear that avaya phone doesn't support 407 Proxy authentication required, instead to send reInvite with credentials it sends ACK and the call is dropped.. I am using 2.6.11 firmware but seen that same thing is with 2.6.12: https://www.3cx.com/community/threads/avaya-9650-9620.40086/ Latest firmware which supporting this model is 2.6.12 and it doesn't work, also with 2.6.11 no luck so what I am asking is anyone has some other firmware for this model of phone so that I can do test, maybe some older firmware. I know that phone in unsupported but what I need is basic thing to work; incoming/outgoing calls and nothing more.