Registration fails suddenly without changing settings

I have a Gigaset N300A IP DECT phone that can be hooked up both to a landline and to Ethernet for VoIP. I have been using it for about 6 years and it has worked well with a number of SIP providers: DIDLogic, Localphone, VoIPFibre, VoipVoip. It is connected via Ethernet directly to my VDSL router.

For the last few months, I have used the last three of the providers mentioned above and they have all been working fine. Then suddenly a week ago, all three stopped working at the same time. In the admin panel of the N300A IP, it says "Registration failed" for all three.

I made no changes to the N300A IP at the time, nor did I make any changes to the router.

I have since tried:
- Rebooting the N300A IP (no difference)
- Disconnecting and reconnecting the N300A IP (no difference)
- Updating the firmware of the N300A IP (which worked, showing that it is connected to the Internet, but it did not solve the issue)
- Re-entering the SIP settings for one of the accounts (no difference)
- Using my VoIP accounts using a soft phone on my Android phone via the WiFi of the same router that the N300A IP is connected to (this works fine)

In the log file from my router, I notice errors such as:
DNS name resolution failure (_sip._tcp.localphone.com)
DNS name resolution failure (_sip._tcp.proxy.localphone.com)
DNS name resolution failure (_sip._udp.sip1.voipfibre.com)
DNS name resolution failure (_sip._tcp.sip1.voipfibre.com)

Since I have never looked at the log before, I cannot say if these errors are new and related to the issues with registration. However, as an experiment, I changed the DNS server settings on the router from 192.168.1.1 to OpenDNS servers, but this also did not resolve the problem.

I am puzzled by the underscores in the DNS errors above - why is the N300A IP looking up _sip._tcp... instead of sip.tcp... ?

Any advice most appreciated!

These are DNS 'SRV' type records not your standard A records.

Take a look here :

https://en.wikipedia.org/wiki/SRV_record

Why this behaviour has changed I'm not sure.

On your IP phone try changing the �sip transport� mode from UDP to TCP or vice versa. Also try rolling back the firmware to the same version being used when the handset was working fine. Worth trying a different router as well.

Edited by deleted (Sat 13-Jul-19 21:05:40)

I'm not sure if this is the same problem as I don't really use my VoIP line so don't pay much attention, but I've noticed a week or so ago that my Gigaset GO (pretty much the same as the N300) seems to drop the rgistration to my sipgate basic account. I've not tried to resolve things, but kind of trying to make a call etc seems to kick it back into life, then a while later it says regitration failed again.

I'm not aware the base or handsets doing any firmware updates recently but I have done a router update, so may have to spend a bit of time rolling back etc to see if I can find the issue.

I fixed it. I noticed that the router had been rebooted 9 days ago and surmised that might also mean the broadband provider had pushed some new settings. I turned off the router's DNS server (which is only required for the router-built-in parental control anyway) and that fixed the issue.

The router's interface is poorly implemented. You can put in DNS server IPs such as OpenDNS or Google but it does not seem to make a difference. The only way to avoid using the in-built DNS server is to turn it off.

What puzzles me still is how I could get the soft SIP phone on my Android to work going through the same router when the N300A IP would not work. But, that is now an academic question - everything works.

Thanks to @JonRennie, @baby_frogmella and @adrenalize_ for the responses.