MO and MT billing are the two methods how carriers charge users.
With MO-billing (Mobile Originating Billing), the end-user is charged for sending a message, thus the billing status is checked before the request to service backend.
With MT-billing (Mobile Terminating Billing) operators charge for the reply-message sent back to the user. In this case, users are billed only when they receive the reply-message. If there are not enough funds on the balance, the reply message is not delivered. Please note, that with MT-billing the billing status is checked after the request to service back-end.
Different operators support different billing methods, for additional information please check the
countries page.