Operating system name and version: Debian
Version of smsd:
Smsd installed from: sources / package repository / from elsewhere...
Name and model of a modem / phone:
Interface: serial / USB / some adapter...

I want the ability to send a sms when someone makes a phone call, for example when someone calls the number then they would get a text saying that this line does not accept phone calls and accepts only SMS messages. Is this possible with SMS server 3?

It is possible. You need a modem which has "MC" memory for missed calls. Then define phonecalls = yes in the modem section in smsd.conf. Create eventhandler script. Every time when the number is called, eventhandler is executed with parameter CALL. The script can get callers number from the message file, and create SMS answer.

See this topic for example:
[answered] when an incoming call, hang up the phone, send SMS that contact you.