mypixmessages.com I believe is a 3rd party service provider. Yeah, vzw uses them, and so does PPC.
PPC may use them to make things simpler, since it doesnt require any changes for vzw phones on their network, maybe it helps them cut down data transfers over their network, maybe they are inexpensive, who knows?
only thing I can think of is that your OTA programming didnt go right when you reset your phone for working on ppc. but then, I am far from being a phone tech, so there could definitely be other reason that I'm unaware of.