AT&T runs transparent proxies on their network that have been known to intercept and mangle tls, and directly manipulate tcp packets on the wire.
Take a tcpdump capture on both server/client side and compare the handshake/tcp headers/negotiated window size. Often you see the server sending one thing, but the client sees another
Take a tcpdump capture on both server/client side and compare the handshake/tcp headers/negotiated window size. Often you see the server sending one thing, but the client sees another