cmb ... on wiki definition, full cone is akin to 1:1 NAT. This is not done by default in pfSense. The default behavior of pfSense is more like the symmetric, wouldn't you say?
This is why there are proxies for SIP. Which work great and one of the many reasons I use pfSense.
If you need full cone, you can set that up in 1:1 NAT area.