VoLTE calls between carriers

I'm wondering if voLTE only works betweens members of the same carrier. The only time I seem to have an hd call is when I'm speaking to another videotron customer. Same goes for other people I know. They only get hd calling when talking to another customer of their phone company. Is this the case or am I doing something wrong? Is voLTE compatible between any voLTE enabled devices regardless of carrier? Or is it only compatible between members of the same carrier?