Asterisk 10 and random calls not going through

We are having a problem that when we call someone on a cell phone, it sometimes rings and rings and then the people on the cell phones cannot hear anything when they answer. The crazy thing is it doesn’t do it all the time. I thought maybe it had to do with NAT, but when I try to disable nat between our SIP provider and us, I don’t hear anything consistantly. I had to move our phone system to a different server and upgraded and now I wish I never touched it, except we had to move it off of the server it was on. When it was at the older version of asterisk, I never had these complaints. We are using straight SIP trunks from the internet through Nextiva. It is not on it’s own VLAN, and we are going through a Cisco ASA 5510. I also used the freepbx distro. Originally I was using

There doesn’t seem to be an actual pattern except it is when we call out to a cell phone (although I have never had that problem with mine).

Please help.