The Aastra 55i should have STUN support, but not on the firmwares released
with the phone initially. Check your firmware version here:

http://www.aastratelecom.com/cps/rde/xchg/SID-3D8CCB6A-FE4A1E69/03/hs.xsl/21
669.htm

For the 480i, though, I think you're SOL.

- Darren