Friends, some short explanations:

There are 2 kind of source address in telephony:

ANI (automatic number identification) - it is used for billing and can't be spoofed for obvious reason (operator want bill YOU but not fictious person). But this number may be a single one for whole PBX in a single company.

CID - caller ID, that is that phone system says about itself.

If you call 911 or 800 number then they will get your ANI in any case - they pay telephony operator for it. (actually both)

The regular customer gets CID ... but that depends from provider and in some cases some DTMF can force provider get back your ANI. And many providers blocks CID beyond allowed range, for single line - the same CID.

Now, Voice-over-IP - there is some signature service in big providers but not any. Moreover, SIP doesn't have these kind of separation. So, you may spoof your CID but what happens then your call is gated to telephony? Depends, but I assume you can spoof CID but ANI can be the same - gateway line.


BTW, there is yet another number in case of transferring call and you original identity can be find via it in case of transfer.

All that is a reason why there is no 911 on Voice-over-IP beyond some big operator. 911 operator can't reliably find your location.