[Users] Adding Alias on Register

Hi, in my setup userid's are the same as phonenumbers in pstn. so when someone registers with userid 012345, 012345 is his phonenumber. so everyone can call him from the pstn and and 012345 is his calling number. my problem is, when someone uses 6543210 phonenumber in his sip client, openser only creates a contact with 6543210 as username. so he could be reached with 6543210 and can make calls with this calling number. since my accounting works with the userid's i've added to the pstn-gw, i solved the problem for outgoing calls with changing the from-url: avp_printf("i:50", "sip:$au@$fd:5060"); uac_replace_from("$avp(i:50)"); but what can i do with incoming calls? could i create an alias for a user when he registers?