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:
- destination phone number is invalid
- no credits on prepaid SIM card
- cellular operator has blocked a SIM card