In general you shouldn't need to fiddle with the firewall for VOIP to work, however there can be issues with NAT connection timeouts. In a quick look I haven't found exactly where the UDP NAT connection timeouts are set in recent OpenWrt releases or what they're set to, however RFC4787 generally recommends 5 minutes though some thread info I came across suggests OpenWrt is set to 3 minutes.
I believe Fritz!OS has a VOIP connection keep-alive setting: see here - the "port forwarding" option set to 5 minutes in the image. You might try setting this to 60 seconds to see whether that helps.
Edit: See also this post.