Operating system name and version: Ubuntu 20.04.6 LTS
Version of smsd: 3.1.21
Smsd installed from: package repository
Name and model of a modem / phone: ID 2c7c:0121 Quectel Wireless Solutions Co., Ltd. EC21 LTE modem
Interface: USB

This happens about once every 24 hour:

2023-04-11 18:24:40,6, GSM1: Checking if modem needs PIN
2023-04-11 18:24:40,7, GSM1: -> AT+CPIN?
2023-04-11 18:24:40,7, GSM1: Command is sent, waiting for the answer. (10)
2023-04-11 18:24:50,7, GSM1: No answer, put_command expected (OK)|(READY)|( PIN)|( PUK)|(ERROR), timeout occurred. 1.
2023-04-11 18:24:50,7, GSM1: <-
2023-04-11 18:24:50,2, GSM1: PIN handling: expected READY or OK, modem answered
2023-04-11 18:24:50,2, GSM1: Modem handler 0 terminated abnormally. PID: 1500447, was started 23-04-11 06:42:00, up 11:42.
2023-04-11 18:24:50,2, smsd: Modem handler 0 (GSM1) terminated while mainprocess is still running. Exited, status 1.

After restarting the service everything works again for a while.

This started happening about two weeks ago and we didn't change anything prior to it.

Possibly the SIM is slightly damaged or about to break. At this point, I recommend disabling the PIN request. If it is activated on the SIM card, it must be disabled, for example, using the phone. The modem process must be set to not process the PIN code at all: pin = ignore.

The problem should go away this way, but it is very possible that the modem and/or SIM will cause similar "no answer" situations later and in other contexts. You should also browse the log backwards if there are "no answer" situations with other commands. Other problems may have been less visible when the PIN problem is so serious that the process has to be stopped.