Operating system name and version: Linux CentOS 4.8
Version of smsd: 3.1.6
Smsd installed from: sources
Name and model of a modem / phone: Wavecomm Fastrak Supreme 10
Interface: Serial

Would it be safe to compile and install 3.1.11 on top of my current 3.1.6? Would there be any changes needed in /etc/smsd.conf ?

TIA.

Upgrading should be safe, but still keep the 3.1.6 as a backup.

Handshaking has changed in 3.1.9. It should work with your interface, but in case of problems there is a setting send_handshake_select = no available.

If you install more modems which are serving the same queue, use the setting trust_spool = no. With single modem this is not required.