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How P2P is different from A2P
P2P is regulated as a consumer service: free inside-network, often free on-net even across operators, and the originator is always a real MSISDN. A2P is regulated as an enterprise service: per-message wholesale price, sender is usually an alphanumeric string registered with the regulator, and the operator's billing system tags it specifically as A2P for monetization.
Why P2P routes attract grey-route fraud
P2P is cheap. A2P aggregators with weak governance discover they can push enterprise traffic through P2P routes by terminating into a SIM-box (a rack of physical consumer SIMs) or by abusing P2P-grade interconnect. The operator sees what looks like one subscriber sending 50,000 messages a day — but charges them at P2P rates and loses the A2P monetization. SMS firewalls are deployed primarily to find and kill exactly this pattern.
Where you encounter P2P as a buyer
You almost never deliberately use P2P for business messaging. If your aggregator quotes you a price 70% below the market rate for a country, they are routing through P2P, and your messages will start being blocked within weeks as the operator's firewall classifies the route. The only legitimate enterprise use of P2P-style routing is two-way conversational SMS with a long virtual number — see /products/two-way-sms — and that is still billed and routed as A2P.