After adding the line
"s1
respawn:/usr/sbin/vgetty ttyS1" to the /etc/inittab, the error message "init: Id "s1" respawning too fast: disabled for 5 minutes" keeps appearing in /var/log/messages, due to too many vgetty processes respawning at the same time.[SOLUTION]
Check if your modem is initialized properly:
If local echo is turned on (E1), there might be some ping/pong going on - check if Q and E are set correctly.
Here's what worked fine for me:
ATQ0V1E0X3&C1&D3