Dial tone! Then fast busy

I’ve got a Sipura SPA3000 behind a NAT which is connected to an asterisk server (1.2.9 I believe…)

I get a dialtone on the phone, and I am able to make calls successfully, etc. The problem is that when I pick up the phone, I only get the dialtone for about 5 seconds before it turns into a fast busy… I’ve looked through the SPA settings and don’t see anything that looks relevant, and other phones connected to the server (although not behind a NAT) don’t seem to have this problem.

Any thoughts?