Hi.

We have a Draytek 2925 (firmware 3.8.4) and are using Horizon as our VOIP provider.

Phone calls in and out are working but a lot of the extra functions (BLF, DND, Call Forward Busy, Call Forward Always & Call Forward Unreachable/No Answer) aren't working.

Horizon advised it's because UDP packet fragmentation isn't being allowed on the router.

I've checked everything on the router and unchecked the "Always pass inbound fragmented large packets" box and ensured SIP ALG is disabled, but this hasn't helped.

Do I need to amend anything else?

Hi bhflack,

Horizon use a Broadsoft solution to run their hosted platform.

The size of the UDP datagram used for the 'side features' such as BLF is directly related to the number of monitored devices. For reference the platform limit is 50 - this is tied to the maximum datagram payload of 65507 bytes.

Based on an MTU of 1500 this would result in the datagram being split into 45 packets.

The above assumes a great network and optimal working conditions - packets arriving in the wrong order can cause a solution to go wibble and present the wrong/no information.

I'd start by checking the MTU settings on your router are optimal https://www.draytek.com/en/faq/faq-connectivity/connectivity.wan/what-mtu-should-i-set-on-my-router/
Check syslog on router for firewall - looking for UDP floods or review settings UDP flood defense (defence) in Firewall -> DOS

There may be a setting on the phone that can help - are they polycom by chance? Try using tcp signalling instead of udp (including for outbound proxy), also worth trying disabling storm filtering in network settings.

Good luck

Cheers
Adam

Hi Adam.

Thanks for your detailed response.

To answer your points:
- the phones are Polycom
- MTU was set correctly as advised by the tool
- I made the other settings (except for changing to TCP)

However a Draytek engineer eventually fixed it for me, which I'd like to share in case anyone else experiences the problem.

Using Telnet to login, I increased the UDP timeout, using the command 2portmaptime -u 86400"
This extends it to a day, the same that TCP was already set to.

Draytek and Horizon were both surprised this worked but it did!

Thanks for your time, Brad.