What is a MTM text message

There are two kinds of messages sent via Messages:

iMessages (blue bubble)

SMS (green bubble)

If a message is sent between two iPhone users, it will usually be sent using iMessage (blue bubble) which means the message is not sent via SMS but rather via a data connection between the sender and Apple's servers and Apple's servers and the recipient.

If a message is sent between an iPhone user and an Android (or other phone user) or vice-versa, it will be sent as an SMS message (green bubble). That's where MTM comes into play.

It sounds like your message is being sent as an iMessage and you are being charged data rates for sending it.

What is the difference between iMessage and SMS/MMS? - Apple Support

It's an SMS text message sent from one mobile user to another mobile user.

Many cellular carriers don't charge for or charge less for such messages, especially if both the originator and recipient are on the same cellular carrier.