asterisk users - Apr 2007 - How to set fromuser in sip.conf so each user gets it's own callerid?

I'm a first time user of Asterisk and have a working setup which I find
clumsy. How can I clean things up to make the dialplan easier to maintain?

My problem
=========I have 6 public numbers that can reach 6 individual users. I have 6
lines like this in sip.conf:

[general]
register =>  31307115622:secret@belcentrale-incoming/622
....
register =>  31307115627:secret@belcentrale-incoming/627

each user registers with something like this:

[siemens1](xanadu-internal)
type=friend
callerid=Theo Band
context=xanadu-thba

[belcentrale-out-thba](belcentrale-outgoing)
type=peer
fromuser=31307115622


My extenson.conf looks like this:
[xanadu-thba]
exten => _+.,1,goto(00${EXTEN:1},1);00 is long distance calls => +
exten => _0[1-9].,1,goto(0031${EXTEN:1},1);local calls =>0031
exten => _0031Z., 1,Macro(dialout,SIP/${EXTEN}@belcentrale-out-thba)    ;NL
exten => _+ZXX.,  1,Macro(dialout,SIP/00${EXTEN:1}@belcentrale-out-thba);INT

My dialplan contains a context like xanadu-<user> for every user, only
to be able to set the fromuser correctly.

This works but I prefer to have one dialplan and set some sort of
variable containing the fromuser in sip.conf for every registered user.
I read the entire march list (that's a lot!) and was also not able to
find a proper search term that covers this question.

Thanks,
Theo