Your supplier is correct. If MiCollab is in LAN mode, meaning you only have a LAN interface on the MiCollab server (look in the MBG dashboard to confirm), then you should be able to point your internal DNS to the MiCollab LAN IP and it should work. If it's in server-gateway mode, with a LAN and Internet IP, then softphone traffic will always use the WAN IP and you must adjust your internal DNS accordingly. Honestly the easiest solution would be to remove the internal DNS entry and just force all clients to use the MBG internet IP. The internal traffic will just trombone right back out from your internet IP back to the MBG, but that won't hurt anything and will save you a ton of headaches.