You should be able to use any online/web-based or local SIP server/device. e.g.

As an example, when configuring your TPControl settings for, use the following Settings in TPControl:

SIP Settings:
Username: [your_registered_SIP2SIP_username]
Password: [your_registered_SIP2SIP_password]
SIP Server:

SIP Proxy:
Connection Type: TCP
Outbound Proxy: Enabled 

Specifically for testing with AMX devices, as they do not support cross-domain SIP calling nor will they register to web-based SIP servers, I use a local Asterisk PBX server running in a VM when testing AMX devices, using relevant UDP or TCP connection type as configured in the Asterisk server.
We are not restricted in our implementation for cross-domain or web-based SIP registration, hence the reference to the online SIP2SIP service, and we also support SIP Video whereas AMX devices currently do not.