[Skunkworks] encrypting sms messages

So am working on an application to encrypt sms messages before they are sent and decrypted on delivery as long as the message is received by the intended recipient by matching the phone no from the sender with that on the recipient's sim card. Question is? 1. How do i get the sim cards phone no. without having to send an sms to another no. which sends a reply. Trying to avoid this method The intention is to eliminate middlemen from reading messages and if they were to intercept the message it would be encrypted. 2. So how easy would it be to spoof the app?