Network diagram http://www.gliffy.com/go/publish/6010719
We have a small-business grade NAT router, Draytek Vigor 2950. Devices behind the router are unable to communicate with NTP servers on the outside. This includes both Windows PCs, and our IP phones. All other services (DNS, HTTP, FTP, IMAP, SMTP, POP) function as expected.
I know that NTP (edit used to be "NAT") uses UDP port 123. I thought I wouldn't have to make any adjustments on the router in order for UDP to function properly; the NAT functionality should take care of it. Our IP phones (and some PCs) use SIP to initiate the connection; they work fine.
EDIT: I decided to test other UDP applications. Using the method suggested at https://support.safesoftsolutions.com/index.php?pg=kb.page&id=639 nothing gets through. I'm confused.
How can I further test?