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@Schmidbauer, Keeping registration on both (kamailio and fs) will be unnecessary overload. Isn't it? 

You can use a database backend (like postgres, mysql, etc...) to store user location information in Kamailio (usrloc module)

Then I have FS query the same user location database to generate the bridge string.

This is just the way I've done it and it works great.

So the registration information is kept in a single database.