It is possible to use pjsip to connect to a 3CX Phone System over the 3CX tunnel?

I will split the question in some sub questions:

1. Is the 3CX tunnel a true TCP SIP proxy?
2. It is possible to use the pjsip to connect to it?
3. How should I set up the account and connection data if this is possible?

Let's consider the 3CX server is running behind the NAT and the public IP address is: 123.123.123.123, tunnel is working on port 5090 and the internal address of the 3CX is 192.168.1.13. the SIP user is user1@123.123.123.123 with pass: userPass and the tunnel has the password tunnelPass.

How should the pjsip account structure be initialized?

1. 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).

1. 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.

2. Not applicable since it's not possible to use an alternative client.