Got my Galaxy S8 from AT&T SM-G950U working with XM. It was a full day of work! The person who gave me the SIM at the store, inserted in while phone was on. It did not work, even after reboot. It kept on saying "Emergency calls Only". He was then blaming that phone may not be unlocked for carrier.
Tried to call XM support and Samsung support, secret keys to debug and diagnose, network setting reset, cache wipe etc. Nothing helped. Finally, did factory reset on the phone and it worked!
Make sure you backup and restore using Samsung "Smart Switch". As a bonus, after the switch, phone got upgrade to Pie version of android via smart switch.
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