Has this been a recent issue, or has the phone never been capable of receiving? My MMSing is also slightly messed up - can't receive with my Nokia E51 on MTN.
I tested the mms again it works (sending and receiving), try *123# request the mms settings, change the settings and restart the phone maybe that will solve it. You can try to mms from another phone with the same sim to see if it is a sim problem. If not go to an MTN store and let them try to solve it for you.