The need for an SBC

[exaultu]

I'm sorry, I'm sure the answer is somewhere out there on the internet, but I just don't understand why I get support calls about (usually YEALINK) hardphones that suddenly decide to stop working and refuse to register with the (off-site) PBX using direct SIP STUN. Firmware updated. These sites never have an SBC there, and installing one ALWAYS fixes it. Customers get irritated and confused on why suddenly they need this device on their network for their phones to work when they somehow worked just fine before. I don't really have an answer for them and I just don't understand what's changing that's causing customers to suddenly need an SBC. I know this has a lot to do with phone(s) surrounding network infrastructure, could it be something as small as a router/firewall updating itself or its configuration? Am I missing something here?

For whatever reason, this pretty much never happens with Fanvil phones and Fanvil phones seem to play a lot nicer without an SBC a lot of the time.

 

[SteveITS]

Quick thoughts: ports not forwarded in the router, the phone IP changes, SIP ALG or other router "feature" gets turned on.

 

[JohnS_3CX]

Hello,

I don't really have an answer for them and I just don't understand what's changing that's causing customers to suddenly need an SBC.

You might never understand it fully because you can't really predict what the customer's network environment will do, that is mainly what causes the problems.

It's not the phones, not the PBX but the network conditions that are out of your control.
Every network environment is different (different ISP, different modem, router, firewall switches etc). You cannot predict how STUN will perform, unless you put certain controls in place to get more predictable behavior.

This is regardless of what brand you use. You might wake up one day to misbehaving Fanvils too, I've experienced that myself so I can confirm that it is both possible and probable.

The SBC can bypass the problems caused by NAT, port management, and SIP ALG (the usual suspects).
The phones communicate with the SBC rather than with the PBX, so they never have to traverse NAT or come up against any other restrictions. The SBC then finds its own way to the PBX and "bridges" the phones with it.

I hope that makes things a little more clear!