You don't need to have the phones in srst mode for this work. This will work even if phones are registered to call manager.
1. Have you enabled multicast pim on voice vlan.
2. What codec are you using between moh-region and remote-phones-region
3. enable 'deb ccm-manager music-on-hold all', remove the command 'moh audio.wav' and re-add it then post the output. We need to make sure that voice gateway is parsing the file.
4. What is the base address/port for your multicast moh server
5. Check which cucm node is your moh server registered with, login to the cucm cli and get the output of the command show perf query class "Cisco MOH Device" when you place the call on hold.