Phonescoop has not mentioned a carrier even once... but they did test it on something...
But it is CDMA... it's not like someone can just activate it on any carrier. Sprint will not allow foreign ESN's on the network. As a matter of fact, I believe Alltel and Verizon are starting to block foreign ESN's as well. The only carrier I know that activates anything are Cricket and Page Plus (MVNO).
you order the phones and everything through myjitterbug.com everything on the phone is branded jitterbug except for the start up animation on the handset which says great call. i think they're intentionally trying to make everything have the jitterbug identity, phones, service, live operators, etc.
i don't think elderly people who don't know much about cellular service to begin with would benefit from knowing their jitterbug phone is actually sold by a company call GreatCall who leases network service from other companies.
and as for true carrier, i'm no sure who GreatCall uses.