ALG or Asterisk problem?

Hi,
i’ve from 2 years a FreePBX 2.9.0.12 with asterisk 1.6.2.20 and a trunk sip where i haven’t problem.
Now i have create a new 2.210.62-5 installation with asterisk 10.12.0. I’ve rewrite all configuration, but the trunk sip can not register if my firewall have the alg sip on. Why there is this problem? Is a Asterisk 10 bug?
Then i’ve this disconnect and reconnect of the trunk every 30 minutes about (some day every 1 or 2 hours or more). Where is the problem?

[2013-01-31 12:29:06] NOTICE[1492] chan_sip.c: Peer ‘XXXXXXX’ is now UNREACHABLE! Last qualify: 45
[2013-01-31 12:29:43] NOTICE[1492] chan_sip.c: Peer ‘XXXXXXX’ is now Reachable. (52ms / 2000ms)

Thank you.

Giuseppe

Your best bet is to disable SIP ALG on your router.