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

The Fortigate and SonicWall are both VPN capable, create a VPN between the two sites, and you will have no more issues such as your describe.

Don’t forget to add the remote network to the Localnet table in SIP settings module.

Thank you for you quick reply, I will try moving the phones from the external address to the local address.