As a result, the Kamailio process is stuck and not able to restart (dead lock).

To resolve such a case i want to utilize systemd watchdog feature

To use this feature I need send  sd_notify(3) with a message string of "WATCHDOG=1" from main process

According to systemd doc only one process to send watchdog

If the $WATCHDOG_USEC environment variable is set, and the $WATCHDOG_PID variable is unset or set to the PID of the current process, the service manager expects notifications from this process.

Additional info at

Could you suggest how to embed this systemd call to the main process?

Sergey