This Python script will let you use your VoIP.ms account to send text messages using SIP. You only need to provide your log credentials and the desired server.
Usage:
sendsms.py [PHONE NUMBER] [MESSAGE]
Messages longer than 160 characters are automatically split over multiple text messages by VoIP.ms
I only tested this with a Sub-Account. It might work with a main account or with IP authentication as well, but this is untested.