I think pretty well all SIP phones also support STUN, which is what you will normally use to handle NAT. The Sipuras (Linksys, now Cisco) can be configured to frig their SIP packets for NAT without using STUN if the external IP stays the same, but it's probably just easier to use STUN always. It will work out what sort of NAT is in use, and take appropriate measures to get SIP and RTP through it without you having to worry about it. I suspect all ITSP's will provide an STUN server, but in theory you can use anyone's STUN server, providing they don't block you and don't mind.