Operating system name and version: Debian 9.4 (Stretch)
Version of smsd: 3.1.15
Smsd installed from: package repository
Name and model of a modem / phone: Huawei Technologies Co., Ltd. K3772
Interface: /dev/ttyGSM0

When I remove my USB modem I get this error messages:

2018-08-07 17:38:09,3, AT0: write_to_modem: error 5: Input/output error 2018-08-07 17:38:19,3, AT0: write_to_modem: error 5: Input/output error 2018-08-07 17:38:30,3, AT0: write_to_modem: error 5: Input/output error

The problem is: When I reconnect the USB modem the error messages keep showing up, they do not go away. smsd does not realize that the interface is available again.

The interface (/dev/ttyGSM0) does not change because it gets automatically symlinked by udev, so that does not cause the issue:

/etc/udev/rules.d/20-gsm0.rules