I think for sure latest ISR-G2 like 2900, 3900,
and older 2600XM, 2800s routers can be gatekeepers/gateways.
look for IOS with ipvoice in the name; those are likely have VoIP features u need.
if you go to there, navigate Search by Technology, then to Voice , IP Telephony/VoIP, finally gatekeeper in features;
or you could check IOS u have for feature.