The mysipswitch is a free of charge & open source web application which allows to use multiple SIP accounts on the same phone. It does the 3rd party SIP registration and, on dial out, you can set prefixes to make your outgoing call with such or such provider.
This is very handy when it comes to take advantage of different VoIP providers offers. There are other functions as well to allow real time call management from a web browser. You can hold/resume, forward & transfer call on the fly.
A forum is in place, if you have any question : My SIP Switch Forum
The source can be found at : MySIPSwitch source
This project has been sponsored by Blueface : VoIP
These details are provided for information only. No information here is legal advice and should not be used as such.