[Users] check_from

Hi! I am new to SER, and have following question. How can I provide that: 1) Users can register only their sip addresses; 2) Users can use their username in sip From: field. In my installation sip username and authentication usernames differ a little. For example, sip username could be 112233, but auth username is sip112233. So, I want to be sure that user that can successfully autheticate as sip112233 will not use 345678 username, and always will be using 112233 in From? I need something like check_from, but this function requires identical sip and auth usernames. Is this thing doable? If so, I will be glad to hear how I should do this in openser.cfg or any hints/links that could point me in right direction. Thanks! -- Timur Irmatov, JID: thor(a)jabber.org.au.