[Users] lookup function for From URI

[unplug]

As I know, we can use lookup function for lookup an alias of an RURI. 
It is for inbound call.  How about the outbound call?  Say, I have an
account (881234) with an alias (24568849).  Someone can call the alias
to reach the account (881234) by using lookup function.  How can I
make outbound call such that the caller number will be replaced by the
alias?  As I know we can use uac_replace_from(display,uri) to replace
the from URI.  But I have no idea how to retrieve an alias from the
>From URI.  Any clue?