It would be awesome if we could display the callerid information of all incoming telephone calls (from a VoIP provider using SIP protocol) on the top of the TV screen.
It could easily done using some SIP software stack that would display callerid information on the TV screen. It would never answer or make calls.
I’d recommend NOT using the SIP protocol. Make it something more widely useful by adding a simple notification UDP port. Send it a UDP packet with message and an expiration time. This then gives you the flexibility to do things like catch callerid (non-voip) with an old modem and send the announcement to the ROKU player(s).
Or even Roku could provide the VoIP service as well for customers. Not sure the market is there to justify the expense to do it, but if simple, would be a differentiator from FireTV. Many older people still want a separate landline phone.