Several commercial Set Top Boxes (with direct IP connectivity or with built in DOCSIS) implement a pseudo SIP client at the STB, the client registers to the same number as the SIP ATA, so the STB receives the callerID at the same time as the SIP ATA, but the STB never answers the call.

No need to "sniff" SIP data or connect the STB directly to the ATA.

I guess you could do the same on MythTV