Walk onto any voice & messaging vendor's booth at MWC and you will see the official, standardised clients and servers. VoLTE, RCS, interoperability gadgets and so on.
But look a bit closer, and have a quiet word with the staff, and you'll get a different story.
If you're not a roving secret policeman for GSMA, you might get shown the more interesting, samizdat goods that are kept "under the counter" for *special* customers....
They've all got a telco-OTT solution (or several). They just don't shout about it yet, mostly. I've seen 3 OTT visual voicemail solutions already, various soft phones and so on.
UTC OTT. Sshhh, don't tell anyone!