[HELP] Calling Number

Hi,
I am new asterisk user and I have no idea how to put something in front of my extension number.
Example the extension is 16000401
I want to send the A (Calling number) in this format 1000011+16000401.
Any ideas??
Thanks

Dial(SIP/100010001${EXTEN:3}@yourprovider.com)
This way you will cut first 3 digits from your extension and you will ad in front 100010001