3rd Party Hosted VOIP issues

I recently switched from another provider (2 bonded T1s) to comcast business class (50/10) connection with 5 static IPs and everything went well as far as the internet connection goes. Unfortunately I am now having problems getting more than 3 of my VOIP phones to connect to my provider IPfone. I have a DPC3939B modem provided by Comcast and it is set to the "hybrid" bridged mode as per the tech I spoke with. My SoHo firewall/router configuration is exactly the same except it is using the new static IP provided. 

I have 6 Polycom IP335 phones that when rebooted the first 3 to connect to the provider's servers and register will work normally. The other 3 that attempted to register will fail and cannot receive or make any calls. Is there some limitation with the DPC3939B that would cause this issue. As I said all network settings are exactly the same as before switching to Comcast so I do not believe it is with my setup but I'm open to suggestions.

For reference the connections goes from DPC3939B - Fortinet Fortigate 60B (static IP) - gigabit switch - IP Phones.

Thank you,

Eric