Need help setting up E911 with bulk911.com

Good morning,

I'm hoping someone can help me figure this out. I had a local VoIP provider, for E911 use, but they are going out of business. I want to try bulk911.com.
To use bulk911.com, you register your phone number and address. To place a 911 call, you call 911@sip.bulk911.com. To place a TEST call, you call 922@sip.bulk911.com.
I'm able to call the SIP URI by setting up a SIP extension that forwards to the appropriate SIP URI. However, bulk911.com uses caller ID (or RPID) for verification, which must match the phone number you have registered with them. When I call my PBXes extension that is set up to forward to SIP URI 922@bulk911.com, the caller ID is always sent as the extension number I placed the call from.
Can anyone help me?

Rob