I am no expert on VoIP but I am learning fast!
VoIP uses specific ports to transmit information - first to register the account with the VoIP provider and then to transmit the voice call in digitised format. These were all open before Clear Comms. introduced their new login system.
VoIPtalk's proxy server is at nat.voiptalk.org on port 5065 which is fine. Port 5004 which is used for RTP is also open. Some of the other ports that are required for VoIP are obviously closed or are retricted.
My Grandstream phone is set to use a non default port for the local SIP connection. The port it uses is still open which is why the old phone works. I suspect that port 5060, which is the default port for VoIP, has been either closed or restricted. That is possibly why my Siemens phone connects to VoIPtalk but won't register. The other interesting thing is that Siemens server for updates is unavailable.
Even though the old phone works, the call quality is back to being poor.
We are going up to see Keith McClure again today to pay for April's connection so I'll see what he says.