no voice for three weeks - but internet fine

[#1]

Not sure, @RobPN , but I don't think Zen offer a VoIP-only service -  I just know you don't have to use their DV service if you are with them if you wish to use another VoIP service provider. (Thanks, TBB forum).

[#2]

Yes, thanks @jab1 , I know I could Google it, and I know you don't have to use their VoIP.

I wasn't questioning whether it was possible to get a standalone VoIP service from Zen, I was just getting at the technicalities of whether it was a proprietary service like BTs.  You can also use any third party VoIP service with BT etc., but they restrict the freedom to use their own DV services with third party equipment which is one of the drawbacks @outcast was pointing out.

Edit: typo

Edit 2: added words in italics

[#3]

@RobPN I believe you can use Zen DV with a mobile phone. You have to use their FritzFon app though,

 https://www.zen.co.uk/help-support/fritzfon-app/ and configure it by connecting to the Fritzbox router. I assume that's necessary to download the correct voip config from the router.

Zen do have a SIP voip product for Business https://business.zen.co.uk/voice/sip-trunking . Our office BB is Zen and a few years ago when I was migrating our phone system to voip I looked at the Zen offering. However their support couldn't explain how I would use a DiD number block with their system. A & A were able to explain perfectly and offered me a temporary block of 2 numbers to test my system before the existing PSTN number port was progressed.

My guess is that their (Zen) residential DV offering is based on the sip trunking but with a 'locked down' configuration built into the Fritzbox router.

[#4]

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@RobPN wrote:

Yes, thanks @jab1 , I know I could Google it, and I know you don't have to use their VoIP.

I wasn't questioning whether it was possible to get a standalone VoIP service from Zen, I was just getting at the technicalities of whether it was a proprietary service like BTs.  You can also use any third party VoIP service with BT etc., but they restrict the freedom to use their own DV services with third party equipment which is one of the drawbacks @outcast was pointing out.

Edit: typo

Edit 2: added words in italics

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Fair enough - although as @MisterW  has given you a more authoritative answer...