P2P (person-to-person) SMS is a message exchanged between two individuals from their own phones; carriers price and route it differently from application traffic.
P2P, person-to-person, is the original use of SMS: one subscriber texting another. It is billed by the sender's plan, sent from a mobile number, and expected to be conversational, low volume and two-way. Carriers use these traits to tell it apart from A2P traffic.
When application traffic is pushed through P2P channels to avoid A2P pricing, carriers detect the pattern (high volume, identical texts, no replies) and filter it. That is why a gateway sends through A2P connections even when the message looks like a simple notification.
At SMSMeteor: messages are never sent from consumer SIMs; the platform uses A2P routes, and a two-way conversation is possible by renting a dedicated number with inbound.