[Asterisk-Users] sip accounts

[Kong]

hi, i facing a problem here. in my sip.conf, i specify a account like this,
[1234]
type=friend
context=from-sip
username=1234
secret=1234
nat=no
canreinvite=yes
dtmfmode=info
mailbox=1234 at default
disallow=all
allow=ulaw

so i am able to login with username 1234 and password 1234

but ther weird part is, i can also register as any number (account) 
without having to specify in sip.conf. thus anybody can just use my 
system to call others.. lets say i do have set that some certain 
account can make some certain calls only.

how can i solve this problem?

thank you.