[Users] problem with uac_replace_from

I am using uac_replace_from to set the Sip display info. The problem is that it only replaces the display when one exists and is passed. It doesn't create one when one doesn't exist. So for example if I use ...

uac_replace_from("batman","");

then place a call from one of my registered user agents to another then the callerid name value is displayed properly as "batman", however if I place a call from the pstn to one of my user agents which comes in with no Sip display into set then it is not generated as batman and added. I need sip display info to be replaced if it does exist, and generated if it doesn't. Any help will be greatly appreciated.