I have around 10 remote sites. My 3CX server is located in a data center with static IP. We have no local extensions. All sites are behind a basic firewall and we use Cisco SPA504G's at most sites (some use SPA 525G). We do not use 3CX tunnel. We have been using 3CX in this scenario for over 12 months with great results. When this was initially setup, all extension were set to "PBX Delivers Audio" to remove the possibility of one way / no audio etc. However, I'm not trying to improve performance and avoid having the 3CX server transcode all calls, so I have removed this option. After playing around with SIP ALG / NAT etc all is working well. No audio problems within sites or site to site. However there is no Music on Hold. When you place someone on hold you just get a couple of beeps rather than the music the 3CX server is setup to play. If I re-enable "PBX Delivers Audio". Music on Hold comes back. Any ideas?