I had a similar issue with my samsung galaxy s7 verizon. The only difference between my issue and the one discussed in this thread is that when i try calling on my phone using another phone, it just says “User Busy”.
i solved it when i found out that i messed up a little bit in my mobile networks setting where i selected preferred network mode to LTE/ CDMA instead of selecting global or LTE/ GSM/ UMTS.
I fixed my issue and now my phone is running very fine.