Can't connect a VOIP phone to 3CX on BT Business hub / Broadband.

[Drew_H]

I'm trying to provision a 3CX phone at work, but It won't register.  Same phone, same settings will register at home on PlusNet or on a 4G LTE router.  Using a Yealink W70B phone, what I have tried:

1) Factory reset the phone, add the sipgate account to line 1, works fine, at work, home or on a 4G LTE router.
2) Factory reset the phone, auto provision the 3CX account. Won't register at work on my BT business hub. Works at home on my Plusnet router. Works on a 4G LTE router.
3) As 2), but add sipgate to line 2. Sipgate won't work anywhere. On the phone the 2nd line has a red phone with crossed circle symbol and can't be selected.

Firewall on my BT hub is set to allow all outgoing and I can't see any setting that could affect it. The Sipgate account works on the BT hub with the same port number 5060 (only in situation 1 though). Any ideas why the 3CX won't work through the BT hub?

[tomas573]

Hello,

This is a classic and frustrating network issue, and it's good you've isolated the problem to your BT Business Hub. Here's a breakdown of potential causes and troubleshooting steps, focusing on why 3CX provisioning fails specifically on that network:

Key Problem Areas:

SIP ALG (Application Layer Gateway):       
This is the most common culprit. SIP ALG is a feature in many routers that tries to "help" SIP traffic by modifying it. However, it often mangles the SIP packets, causing registration and call issues.
BT Business Hubs are known to sometimes have SIP ALG enabled by default.
Port Forwarding/Firewall:   e-zpassnj com
While you've opened outgoing traffic, 3CX requires specific ports to be open bidirectionally.
Even if you've allowed all outgoing.

Best Regards