Also, the 'max_contacts' setting controls the number of concurrent contact URIs that the registrar can hold for registered clients. For instance, if a user has two phones in two different places, a setting of 2 or greater will allow them both to be reachable in a non-mutually exclusive way.
It has nothing to do with limiting calls to the PSTN.
If you want to limit concurrent calls to the PSTN, you are going to have to use an AVP or a custom database interaction or an outboard script called from the route plan in order to flag a call as being open or closed, and reject subsequent calls if one is already in progress. This stands to be mostly reliable, although it will have trouble if OpenSER dies in the middle of an open call and never detects the BYE or CANCEL corresponding to it, although using a persistent database entry that stores the Call-ID (GUID) can even let you get around that to some extent.
-- Alex Balashov Evariste Systems Web : http://www.evaristesys.com/ Tel : +1-678-954-0670 Direct : +1-678-954-0671