Avaya calls over VPN dropping after 30 seconds

[ferna236]

I hope this is the right place to post this:

I am having trouble with site-to-site VPN setup. Each site has an IP office 500. I seem to have set up the tunnel but when I try making calls to the other end, the calls disconnect after exactly 30 seconds. I have tried googling and what not, but haven't come up with a solution. I've tried disabling H.323 inspection on firewalls at both ends (A Cisco ASA and a SonicWall) but with no results. No matter what I try, the problem doesn't seem to go away. Is there something I'm overlooking. Since the calls disconnect at almost exactly 30 seconds, I'm guessing the problem is with our Avaya systems and not with the firewalls.