Please answer below questions:
1. What the gateway model and IOS version?
2. Are you using SIP Trunk or PRI lines? please post you gateway configuration.
3. Do you have gateway device pool configured with MRGL?
4. Which codec you are using between Phone device pool and gateway device pool?
Ensure that the region between the MOH server and the GW is G711 if you want it to play as G711, otherwise you need to enable G729 codec under MOH service parameters. Try adding software MTP resource under MRGL and enable MTP on SIP Trunk.