Someone

============sip.conf=================
[2000]
type=friend
secret=2000
context=default
username=2000
host=dynamic
canreinvite=no

[arati]
type=friend
host=dynamic
secret=arati
context=default
username=arati
disallow=all
allow=ulaw
mailbox=8000

============extension.conf==============

exten=>2000,1,Playback,transfer
exten=>2000,2,Dial(SIP/2000|25|r)

exten => 8000,1,Playback,transfer
exten => 8000,2,Dial(SIP/8000|25|r)