VOIP/SIP audio issues

Let me lay out our setup:

Comcast 1gig down, 40 meg up. Modem is technicolor CGA4131COM. We have 5 static IP addresses.

Next, physical firewall Sophos SG-125 UTM9. We Have about 20 tunnels to various facilities across the country.

• eth0 External
• eth1 internal 10.13.13.1/24
• eth2 vlan 100 10.13.100.1/24

The phones are scattered throughout 100.1 and 13.1 which I know is far from ideal. These devices are also in bridge mode which is further from ideal.

Voip provider: Vonage, majority of the devices are T46S

We're having issues with call audio, and more specifically audio dropping. It's never a very long period when we lose audio, maybe a word or 2 from a sentence and random intervals. Vonage doesn't have specific SIP servers per say but rather dnsnaptr for easy failover I assume. Vonage uses ports 10000:30000 for RTP and 5060/5061 for SIP and SIP TLS, respectively. I have traffic on those ports allowed, any <-> any. Here's the testing I've tried so far.

1. Bypass firewall, connect phone directly to modem. Issue still persists. It is worth noting that calls to/from external #'s are still experiencing call interruptions at random intervals.
2. Brought IP phone home, tested call quality and everything was relatively flawless. No interruptions.

Comcast seems to think it's a vonage issue, vonage seems to think its a comcast issue. Comcast is sending out a tech tomorrow, but their initial findings suggest that nothing weird is going on.