These routers, DPC39xx, are currently blacklisted (2016 to present).

Phones randomly and intermittently don't respond to invites, sending calls to the failover or not at all.  More rarely phones will have the inability to place outbound calls.  Several OnSIP customers that experienced issues with this router replaced the router.  At this time, there is no workaround and we do not recommend it for OnSIP use.

The only solution we have seen is to either replace the router and gamble on a new Comcast router that maybe has fewer issues or, better yet, to place the router in 'bridge mode' and add your own router.

Comcast >> Cisco DPC39xxB in Bridge Mode >> New Router

Technical:

Despite the phones sending keep-alive packets even as quickly as every 10 seconds the router does not seem to keep the port open.  Additionally, the router will reassign recently used ports to other phones causing the phones to respond with "404 Not Found" to inbound calls because the INVITE is not for that phone.  Looking at the registrations, the ports tend to be in the 109x to 110x range.

Edited: 08/25/2020