It seems VoLTE with T-Mobile US has broken on the 3T. Calls will come in as non-HD and SMS will not send with it enabled. Multiple confirmed reports on both 3.5.3 and 3.5.4.
Anyone experiencing this issue?
If so, please post here and in the bug thread: https://forums.oneplus.net/threads/volte-does-not-work.480271/
Additional discussion: https://www.reddit.com/r/oneplus/comments/5jdlz3/volte_breaks_sms_on_tmobile_us_3t/