Thanks for the replies - I have now finally got it all to work!

Thanks to the suggestion (above) to try a SIP client on my mobile phone, I used the Android Linphone app which kept connected over several days and strongly implied the A&A service would always work properly for my number with the right equipment.

So I gave up further tinkering with the settings on the FritzBox 7530 - despite several here saying you were using this for the A&A service. I found a cheap eBay used FRITZ!Box 7590 (not the newest version, but running the latest FRITZ!OS: 8.20). It looks as though it might have originally been supplied by another ISP, but anyway, following the relevant A&A help page, it was straightforward to configure and test it.

And it works! Since setting it up yesterday, the telephony connection has remained stable for over 24 hours and the fibre broadband with Zen has been as stable as usual. The Fritz!Box answerphone and missed call messaging work as they should, too.

One quirk, which I found using the Fritz!box diagnostics, was that it seems to disconnect and then reconnect in the middle of the night because the system enabled automatic clearing of the internet connection in the Connection Settings of the FRITZ!Box. It seems the reason is: 'Clearing internet connection briefly to prevent the internet service provider from disconnecting you in the immediate future.' Didn't seem to do any harm, just meant a few seconds downtime, so I've left it alone:

Yes, a recent update for A&A says they will be raising the price of SIP number services, 'brought about by the increase in support burden for these services.... Support of lesser-technical users ... makes it difficult to make the service – originally conceived and priced expecting only highly technical users - profitable, and as many people are reliant on it, we want to make sure it is sustainable so that we can continue to operate it into the future.'

As a kind of semi-technical user - I seem to have contributed to this extra work. Having long ago hear a (highly technical) IT manager praise the A&A service, I now understand why.

Thanks for the update. Pleased you were able to isolate the problem and resolve.

I'm having exactly the same issue with my 7530 AX. I can't work out what the issue is, it connects and works fine but the fails with 403 some time later. A&A say it is t doing the re-register correctly, that is not doing it in a timely manner, I think their default is you must re-register within the hour but this isn't a configurable setting in the FB.

In reply to a post by JAPrufrock:

I'm glad to see this, and would be interested in any tips, as my attempts to do this using Andrews and Arnold VOIP on my FB7530 keep failing. My broadband - FTTP - is with Zen and very stable but I recently ported the number to Andrews & Arnold as Zen wanted to charge £7.50 a month for a line we rarely use for calls but is still useful to share with people happier to call on a landline.

I've followed the guidance on their site, and it works for while (from 8 minutes to 2 hours) but then fails: with the message; 'Registration of internet telephone number **** failed. Remote site reports reason for error 403'

The Andrews & Arnold helpdesk has been very patient but unable to sort this: they say it could be the router that's faulty, though the Zen service was fine for 2 years. The FrtizBox helpline suggest it's a provider issue as the connection works for a while. I asked Zen if they've any ideas - has the number ported properly? - but (unsurprisingly) they say they can't help.

Can anyone suggest what might be wrong? I've rebooted the router to no avail. I could do a factory reset but fear I'll mess up the home network in the process, despite recording all settings.

I am also with Zen FTTP for broadband and A&A for voip - and have been for a few years. I have used Fritzbox 7530, 7530AX and 4050 with the same settings and all have been fine.
The main difference to the settings on the aaisp sitar:

Use telephone number for registration UNCHECKED

Always register via an internet connection UNCKECKED

Also on the Fritzbox Support Page (Click Help & Support at bottom of menu frame, ten click support at bottom of help page):

Upstream VoWifi Prioritization for epdg.epc.*.3gppnetwork.org

Enable Prioritization box ticked.

Hope this helps.

Thanks, that gives me hope but I am still having issues with your suggested settings. Can you let me know what you have in the different fields?

(I have used 01234 567890 as an example in the settings below)

Under the telephone number I have:-

Telephone number for registration: +441234567890
Internal telephone number in the Fritz!box: 567890

Account info:-
Username +441234567890
Authentication name: <blank>
Password: <from SIP password in A&A portal>
Registrar: voiceless.aa.net.uk
proxy server: <blank>
STUN server: <blank>

Telephone number format:-
Country code: None
Area code: None

Relay outgoing emergency calls:=
Relay outgoing emergency calls without prefixes: checked
Relay special rate numbers without prefixes: unchecked
Telephony provider with North American number plan (NANP): unchecked
Relay local telephone number in international telephone number format: unchecked

Use telephone number for registration: unchecked
Always register via an internet connection: unchecked

Contact internet telephony provider via: IPv4
Transport protocol: Automatic

The Authentication Name should be the same as Username.

I think the remainder should be ok.

I e added that as well but still breaking. When I go back in to edit and re-register I notice that the username and Auth fields have the number without the +44 on it for some reason. I edit it and put it back before saving, it registers but then breaks again a while later.

I'm getting close to having to buy a separate QTA device for VOIP which is a pain as I moved away from Zen to A&A to save money!

Right. I suspect the problem may be caused by using the Zen supplied 7530 configured as supplied by Zen. This means it will regularly check in with Zen via TR-069 for config changes/updates and this may be causing the settings to change.
I have never used a Zen supplied Fritzbox in this way. I have always set them up myself from scratch thus removing any unwanted changes emanating from Zen. As Zen use TR-069 to configure their own VOIP settings I strongly suspect this is causing your problem.

Ah, what would the solution be in that case? Is it possible to change some settings around that, if I factory wipe it, would I still be able to connect up to the Zen service? (apologies if daft question)

Actually I can see an option under Internet -> Provider services called "Permit connection to your internet provider over USP (TR-369)"
Looks like TR-369 is a consumer version of TR-069? I wonder if I turn this off it will resolve things...not sure what else might not work though?