If you experience the following problem:
(example of modem log entry):
Tue 2021/01/04 00:20:12 gammu-smsd[11255]: SMS sent on device: "/dev/ttySMSA0" status=350, reference=-1
Error 350 means that the message was rejected by a cellular operator. Possible reasons:
- the destination phone number is invalid
- no credits on prepaid SIM card
- cellular operator has blocked a SIM card
This error may also occur if your SIM card does not support IMS service - in most 4G/5G networks that's a default method of sending SMS, and if not enabled on the SIM, sending and SMS may result with this error.