[Serusers] call routing problem

Hi I am mapping external numbers to internal ones in a db, eg 08450040600 maps to 040600 So when a call comes from pstn to 08450040600 it hits the ip phone on 040600, all well and good. Now what I am testing is deploying asterisk for other features like call pickup and all. I have managed to create separate contexts in asterisk based on company ID (i.e virtual PBX) assigned from a DB. And when a call from IP phone A (company id =cid=500) calls out the call goes via ser (thats where all IP phones are registered) to asterisk, and then out to ser --->pstn (was gonna go out via asterisk - pstn, but decided to keep it all within ser). This is fine, because asterisk is in the loop. Now what I need is for inbound calls to that IP phone to also be routed into asterisk, and then to ser and to the ipphone, that way I pull all calls for each company into a asterisk context for all its pbx functionality. The problem is that when 0845 is called from pstn, I cant setflag on it, using is_user_in, since From = pstn dialing number, and To = 4408450040600, and not 040600, any ideas on what I can match on so that I can set the correct flag and divert to asterisk. Iqbal