".... Nathan Freitas, a fellow at the Berkman Center for Internet and Society at Harvard University explained that the carrier may have “details of whom [was]texted and when” but “the actual text is what is really hard to get, if not impossible” from the carrier.
The Boston Globe reported that carriers, including the four biggest in the country ‑ AT&T, Verizon, T-Mobile and Sprint ‑ have publicly confirmed that they delete their copies of messages after delivering them......"
https://news.law.fordham.edu/jcfl/2016/06/02/cell-phone-forensics-powerful-tools-wielded-by-federal-investigators/