We have our fiber provider as our primary ISP. We have Comcast as our secondary ISP and our VoIP provider. We can setup the failover between the two ISPs. We also have 19 VoIP phones that are on a dedicated line with Comcast as a converged network with GIG through-pass on the phones. The phone's are separate from our network and only requires a POE switch. Can we use our Fiber ISP as the data data line through our VoIP phones even though the phones are on Comcast's dedicate network? We'd like for the users with phones to connect to the phone for fiber data and not Comcast's coax data. At the same time, be able to use Comcast's VoIP service.
Depending on how your network is set up, this is certainly possible.
You can configure the phones to pass VoIP on a dedicated, tagged VLAN and traffic from the internal switch untagged - this is what I'd recommend. From the VoIP VLAN you can then route to Comcast, depending on your router/firewall type.
Without VLANs, you could identify the phones in some other way (e.g. reserve IP addresses from a certain range on DHCP) and put a policy route for this range on your router/firewall. In extreme, you could even use a common address pool and put a policy route on the protocols used for VoIP. Note that this leaves your phones very vulnerable within your network.
Without policy routes, you could set up static routes for the VoIP gateway via Comcast WAN while the rest uses the default route via fiber.