It only works specifically with the Nexus 6 (not other Google phones), apparently for technical reasons involving network-switching capabilities. Possibly that's BS, but assuming it's legit I'd imagine they'll try to get the technology into other makers' phones in the future.

Though even if they didn't, Google doesn't have a monopoly position in phone hardware so it's not clear antitrust would come into play.

First of all WiFi calling is still a specialty thing requiring custom firmware (see: you cannot BYOD on Republic Wireless)

Second of all the N6 is one of the few phones with dual GSM and CDMA radios, and T-Mobile/Sprint are GSM/CDMA, respectively.

Thats a choice though, the Sprint iPhone also supports GSM/CDMA (as does the verizon one, and the unlocked one too)

Product synergies only becomes an antitrust problem when the company has a monopoly position.

I think that would require them to have a monopoly on the sale of smartphones and use that to make entry to the new market (mobile carrier).

I wouldn't think so no. Google is not a dominant player in this market (cellular carrier).

It's trying to become a monopoly in various markets.