This is akin to the BCP 38[1] problem with ISPs. I suspect few SIP-based telcos do validation of originating numbers today.

STIR/SHAKEN has the potential to help here, but there are still shortcomings, e.g. when originating calls with source numbers obtained from other carriers[2].

1. https://tools.ietf.org/html/bcp38

2. https://support.bandwidth.com/hc/en-us/articles/360025664313...