We have almost reached (20)10s and it has been 15 years since GSM took fire on it. But do we still have peer-to-peer IP connections in GSM/3G networks? As I have understood mostly phones are behind IPv4 NAT. Piping traffic through server is always possible, but if it's unnecessary, why should we do it.

What options are available to form p2p connection:

- UDP piercing?

- SIP? (does it need a special settings in the phone?)

...or even...

- Data calls between phones

Where I could find more material on the matter?