My SIP provider needs a user id prefix attached to outbound calls in the form 99999999# in order to use static IP based verification. They use the user id prefix to lookup whether the outbound IP address is allowed.
How can I prepend an 8 digit number and '#' to my ourbound calls?
Per the SIP provider's website:
You will need to prefix your outbound calls with this user id order to authenticate your account for outbound calls from the IP specified previously.
For most systems you will need to setup a dialplan to add your user id on outbound calls. If you are not sure how to do this, please contact your PBX vendor.