LINK to this???
They have to comply with the court order
I've made multiple google searches using keywords relevant to the shooting and including "call history", cellular carrier", "phone service", etc and can find no mention of the cellular carrier being mentioned or any claims that that the carrier refused to release records under court order or warrant. No mention of it at all.
I did find links concerning Apple, posted above, relating the the FBI not contacting them withing 48hrs and Apple offering to give advice on getting information from the phone but still unwilling to hack their own system, but nothing relevant to the carrier.