FRITZ!Box 7530 as DECT base

[#1]

Does anybody please have experience to help me sort out my VOIP set up??

I have a Yealink SIP-T31P phone plugged into the Plusnet Hub Two router which works fine.

I have a FRITZ!Box 7530 set up as an IP client acting as a wi-fi extender+switch in another part of the house which also works fine (via powerline).

My problem is trying to connect a DECT handset to the FRITZ!Box (FB) as a DECT base station. I have registered the handset to the FB and it pairs with it OK. But it won't ring or dial out (engaged signal).

The FB overview page shows "Telephony: one phone number active, none registered". The System Event Log shows registration failed "Reason for error: DNS error".

I have tried every possible solution from the FB Knowledge Base and internet searches, messing with port forwarding, fixed IP addresses and lots of router settings without having a clue what any of them mean and without success – the number of computations of combinations I could change would probably be on a par with winning the Lottery! I have no idea where the blockage is and am not at all network savvy.

Is the answer much simpler? Would it help to register the Yealink with the FRITZ!Box perhaps? Do I need to register the handset as an extra device with the VOIP provider? I don't want another extension though, nor to mess up the existing set up – I just want both phones to ring at the same time and to be able to dial out or answer on either.

[#2]

@jric 

I have a FRITZ!Box 7530 set up as an IP client acting as a wi-fi extender+switch in another part of the house which also works fine (via powerline).

How is the 7530 configured and connected ? I suspect as a wireless AP (DHCP disabled) and connected via a LAN port ?

Most routers with built-in VOIP facilities will assume that the voip connection is made via the WAN. In your case there is no WAN and so it can't connect

[#3]

@jric 

Can't really help but have you tried unplugging the Yealink device and then trying the FritzBox?

Brian

[#4]

@jric further to my earlier reply, it seems the 7530 MAY be able to operate its voip client when in IP Client mode.

Use the option 3 here https://fritz.com/en-pl/apps/knowledge-base/fritz-box-7530-ax/177_Conducting-calls-with-the-FRITZ-Bo... to 'setup automatic port forwarding' which should keep the NAT session in your main router open and thus avoid the need for any port forwarding on that router

I still dont like the 'Reason for error: DNS error' , that implies the 7530 cant resolve the url for the voip server. 

[#5]

We have 3x SIP / VoIP accounts, here are a few items to consider :-

Both the Yealink SIP-T31P and the FRITZ!Box 7530 will be configured with your SIP / VoIP a/c,
they both have to register.

To use two VoIP devices, make sure there is not a clash of inbound ports,
i.e. the two VoIP devices need to use different inbound ports.

This reads like FRITZ!Box 7530is being used as a Wireless Access point, so DHCP & Firewalls & NAT should be disabled.
Double NAT needs to be avoided at all costs with VoIP, it is a primary cause of one way voice issues.

The primary Hub / Router will be offering the DHCP service and giving out an IP that has a valid DNS.

[#6]

MisterW: Overview shows 'Internet connected via WAN', and 'LAN connected LAN 1'... Telephony DECT Monitor even has a pretty little picture of the handset!

[#7]

In that case as @PhilipHeyes says you need to make sure both sip devices are using different ports. Disable any port forwarding on the main router, disable stun on the Yealink if its enabled. Set both vothe dealing and 7530 to use 'nat keep alive. For tteb7530 option r in my earlier post. 

That way the NAT translation in the main router will ensure that different public ports are used

[#8]

That DNS error that was mentioned catches my attention. What is that about ?

We keep STUN Enabled on all of our VoIP phones & I use the SIP / VoIP providers STUN server.

I agree Router Port forwarding is to be avoided this is the route folks follow when they are totally lost
and have failed to configure the VoIP devices correctly or failed to allocate different inbound ports to each,
or have 2 routers involved and the devices on the 2nd router do not work.

[#9]

@jric   

I have a Yealink SIP-T31P phone plugged into the Plusnet Hub Two router which works fine.

My problem is trying to connect a DECT handset to the FRITZ!Box (FB) as a DECT base station. I have registered the handset to the FB and it pairs with it OK. But it won't ring or dial out (engaged signal).

The FB overview page shows "Telephony: one phone number active, none registered". 

Have you registered your VOIP account in the Fritzbox telephony section ?

Telephony /Phone Numbers/ New Phone Number.

You don’t mention it in your OP.