Remote office connection problem

I have been having a problem with my Asterisk setup with a remote office. The remote office keeps having problems with calls dropping, call forwarding, and the their ring group does not always ring all of the phones.

I have tried changing the firewall out at the remote office from a Sonicwall TZ 170 to a FortiGate 60C, updating firmware, and opening ports on the firewalls.

At this point I have no idea what else to do. I am new to Asterisk and FreePBX and my company want to try out Asterisk to see if it could work for a small office.

Please let me know if you need any more information.
Thank you for any help you can provide.

Asterisk Version -
FreePBX Version - 2.10
OS - CentOS 5.8

Remote office
Internet - Verizon Fios 35 by 35
Firewall - FortiGate 60C

Server location
Internet - Verizon Fios 35 by 35
Firewall - SonicWall NAS 2400