No and supposedly yes. Right now There are multiple standards and not all use the same standards. IMS is robust to allow this stuff
Good ask, maestrobih. Tidbits is on point here - right now, T-Mobile Video Calling can only be completed between capable TMO devices either over LTE or WiFi - however, as more companies incorporate this technology into their networks, we hope our users will be able to video call anyone who's got the functionality, regardless of the company they use for service. You can find a few more useful FAQ here: T-Mobile Video Calling FAQs.