SIP over TCP?

Hi there, this is more of a general question: has anyone out there implemented SIP over TCP? Pros/Cons? We currently use UDP for our SIP implementations but are considering a switch over to TCP due to firewall restrictions outside of our control.

Any feedback would be greatly appreciated.

Thanks ahead of time.


PS: I read somewhere that Asterisk SIP over TCP support is still being developed and is not widely available. Is this true? Anyone knows? Please advise.