I have noticed that the message ID from MMS replies do not match the one that was provided from sending.
For example, after sending an MMS: the message ID we get back is something like - jo7zm83cga2c277_M.vmmm.mmsc.telstra.com
And if I reply to that message, the message ID that I get in the payload to notifyURL has the message ID - QFRO4ApiA0000045066
This only happens when we reply with an MMS (ie. with an image attached). Replying with an SMS gives the correct message ID.
The problem is, without the correct message ID, we can't match the replies back.