I used to work for a company which wrote & sold telco switch management software. Mostly is was for automatically calculating the Least Cost Route, given a list of carrier price sheets and rates for prefix bands.
We had a few local clients with slightly complicated setups, so we got to implement some matching logic for Call Data Records. Their local end had three switches attached to external trunks (to other carriers) and trunked to each other, and a few digi-boxes which voiped to (say) Afghanistan (telcos always use Afghanistan for examples, since it's the first country in the price sheet).
They would list a cheap per-minute price to +93, accept incoming calls & terminate them at the digi-box (closing the CDR & generating a revenue event). The remote digi-box would then start a new outbound call (and CDR) from their partner's facility and (hopefully) get to a subscriber line without going through too many carriers.
The trick is (and we never asked or found out) is that most of the time the remote digi-box is actually a carousel of SIM cards with unlimited local calls. The carousel is used to automatically distribute the calls over the SIMs to impede fraud detection by the mobile carrier.
These setups are pretty common & are called grey routes.