No go for STUN.
Registration works right away on my Snom phone. here's the log
15/11/2012 17:04:34 [DEBUG0] PHN: Trusted IP Addresses: udp:67.205.74.164
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: recv 100 REGISTER (3: 386d43b61c46-97fmu2031zs6) <- udp:67.205.74.164:5060
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: registration report 100/0: ''
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: recv 401 REGISTER (3: 386d43a61c46-97fmu2031zs6) <- udp:67.205.74.164:5060
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: clear message repetition 1000005 386d43a61c46-97fmu2031zs6
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: sip::process_auth Identity=0
15/11/2012 17:04:34 [WARN ] PHN: SIP: process_registrar_packet: 401 needs 128 bit nonce
15/11/2012 17:04:34 [NOTICE] PHN: SIP: process auth:Match challenge for user=MYUSERNAME, realm=montreal.voip.ms
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: Routing to explicit plan udp 67.205.74.164 5060
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: route pending packet 1000007: entry=udp 67.205.74.164 5060 all 3
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: send REGISTER (4: 386d43a61c46-97fmu2031zs6) -> udp:67.205.74.164:5060
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: result of get_ip_adr:67.205.74.164 192.168.1.210
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: result of get_ip_adr:67.205.74.164 192.168.1.210
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: recv 100 REGISTER (4: 386d43b61c46-97fmu2031zs6) <- udp:67.205.74.164:5060
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: registration report 100/0: ''
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: recv 200 REGISTER (4: 386d43b61c46-97fmu2031zs6) <- udp:67.205.74.164:5060
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: clear message repetition 1000007 386d43a61c46-97fmu2031zs6
15/11/2012 17:04:34 [CRITIC] PHN: SIP: Registered at registrar as
MYUSERNAME@montreal.voip.ms (Expires: 180 secs)
15/11/2012 17:04:34 [DEBUG2] GUI: If_Registration(REGISTRAR, CONFIRMED, 0,
MYUSERNAME@montreal.voip.ms,OK)
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: Registration Metrics: 200, sip:MYUSERNAME@192.168.1.210:2048;line=uu1qy1q0, 180, 300
15/11/2012 17:04:34 [DEBUG2] PHN: SIP: registration report 200/0: ''