- It is not a SIP Proxy in the same sense as a SIP Proxy Server. It repackages and sends all media, SIP and other related traffic over a single port (Hence, the tunnel).
The 3CX softphone has this built in. The 3CX Tunnel Proxy is simple a relay for tunnel traffic so that any voip traffic on a network can use the tunnel (and not just the 3cx softphone that has it built in).
No, only the tunnel proxy of softphone is built to connect to it. To make anything else connect to it would require custom programming and analysis of how it worked. It's theoretically possible but probably not worth your time.
Not applicable since it's not possible to use an alternative client.