Finally I changed the modem to another TRM240 and there was no problem - everything works.
So this modem is generally working until a call comes in. If this problem occurs again on a different modem - is there anything I could do? Like reduce the serial baud rate?
By the way, the modem firmware is the same.
Thank you.
Feb 06 08:23:51 raspberrypi ModemManager[472]: <info> [modem0/call0] call state changed: unknown -> ringing-in (incoming-new)
Feb 06 08:23:53 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:23:55 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:23:57 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:24:04 raspberrypi ModemManager[472]: <warn> [modem0] couldn't load call list: Serial command timed out
Feb 06 08:24:06 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:24:13 raspberrypi ModemManager[472]: <warn> [modem0] couldn't load call list: Serial command timed out
Feb 06 08:24:15 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:24:17 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:24:24 raspberrypi ModemManager[472]: <warn> [modem0] couldn't load call list: Serial command timed out
Feb 06 08:24:26 raspberrypi ModemManager[472]: <warn> [modem0] couldn't load call list: Unknown error
Feb 06 08:24:28 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:24:30 raspberrypi ModemManager[472]: <warn> [modem0] couldn't load call list: Unknown error
Feb 06 08:24:32 raspberrypi ModemManager[472]: <warn> [modem0] couldn't load call list: No carrier
Feb 06 08:24:35 raspberrypi ModemManager[472]: <info> [modem0/call0] call state changed: ringing-in -> terminated (unknown)
Feb 06 08:24:35 raspberrypi ModemManager[472]: <warn> [modem0] unexpected incoming call to number 'n/a' reported in call list: state active
Feb 06 08:24:44 raspberrypi ModemManager[472]: <info> [modem0/call1] call state changed: unknown -> ringing-in (incoming-new)
Feb 06 08:24:48 raspberrypi ModemManager[472]: Cannot read from istream: connection broken
Feb 06 08:24:48 raspberrypi ModemManager[472]: [/dev/cdc-wdm0] QMI endpoint hangup: removed
Feb 06 08:24:48 raspberrypi ModemManager[472]: <info> [modem0] connection to qmi-proxy for /dev/cdc-wdm0 lost, reprobing