Well, I can't figure what exactly do you mean with WIFI calling. As far as I know Republic Wireless force calling via WIFI whenever is possible but it is locked to the RW network and cell calling comes as a subsidiary.
Speaking of internet calling in general, you can use it on any Android phone. You must install one of the sip clients as Csipsimple or Sipdroid. In previous versions (ICS, JB) you could use native sip client which is integrated into the system. Motorola removed one file from the system which is essential for the internet calling though (android.software.sip.voip.xml) but you can get it back if you're rooted. But this is not the only problem with the native sip. In KitKat is somehow damaged and using it causes massive voice distortion which is allegedly result of inappropriate echo management. The malfunction is one of about 300 which should have been fixed in 4,4,3 version. It would be nice if someone, who already officially upgraded, tests the sip functionality.
As it has been announced Google plans to integrate his voice app into Hangouts in 5.0 and therefore offer voip as its own standard telephony. It seems that with further development of telecommunications (LTE Advanced) the mobile phone loses its last trad functionality - there will be no more voice transmission, voice will be transferred as any other data via internet from initial base station to the end one.