Thanks Brian and Gad,

I have stun set and if I do a 'sofia status profile internal', I see
the external IP address of the 3102 ATA, so I assume that stun is
working correctly on the SPA3102.

These are the options that I have set (according to the 3102 manual).

• Handle VIA received: yes
• Handle VIA rport: yes
• Insert VIA received: yes
• Insert VIA rport: yes
• Substitute VIA Addr: yes
• Send Resp To Src Port: yes
• STUN Enable: Choose yes.
• STUN Server: stun.freeswitch.org

I assume that is all is needed?