Which ATA?

I have just had Gigaclear fttp installed.
Now I want to ditch BT landline, so I have signed up for Sipgate.
On my Gigaclear router, I have a lan cable to my pc.
To use my existing LL phone, how do I connect the ATA?
Does it have to go into the router, or could it connect to one of the free ethernet sockets in the wall plate?

Its not clear from your post as to what the "free ethernet sockets in the wall plate" are - I take it these are something to do with the service provision from Gigaclear. I presume this will be before the WAN side of your router.

I would suggest that you connect your ATA device to your local Local Area Network (LAN). You want it to be sat behind the (relative) security of your router.

In your case it sounds like you LAN consists of your computer plugged directly into the router, so plugging the ATA into a spare router LAN port would appear the way to go.

There are I believe three ethernet sockets underneath (and inaccessible to me as being disabled) the wall plate “box on the wall” whatever it is called.
I only have one router LAN port, current connected to my PC.
Do I need one of these
TP-Link TL-SG1005D, 5 Port Gigabit Ethernet Network Switch

https://www.amazon.co.uk/gp/product/B00ZOOJXEG/ref=p...

to plug my PC and ATA into?
If so, I am looking for a recommendation of a suitable ATA.

Do you still want to use a corded handset or move to cordless/DECT?

Moving to DECT with a Gigaset unit will offer lots of opportunities ... any of their units based on the N300A_IP usually called S450GO or S850GO will work well.

I am also on Sipgate and used a Cisco SPA122 as my ATA for about a year so I could continue using my DECT BT cordless phones. The SPA122 does require a bit of configuration for things like caller display to work and it also sometimes needed a reboot. I have now switched to a Gigaset N300 IP base station plus a number of handsets - so far it has been 100% reliable. You will need to use a switch like the one you linked to if you only have one router port.

Yes - any gigabit switch will be okay. If funds permit might I suggest you consider an 8 port switch - you will be surprised as to how soon you can fill them up.

As to your ATA query, it depends on whether you wish to continue using your existing telephone setup and just add the ATA device to bridge it to your LAN or whether you are considering a wholly new DECT phone system.

As to the former, I use a Grandstream HT802 ATA which supports 2 VOIP lines, is about £40 and is very capable, but frankly for the novice it has so many configuration options it might be somewhat overwhelming.

If you do decide to purchase one, I found this page helpful in configuring it.

If updating the whole system, the already suggested Gigaset N300A IP has the capability of connecting up to 6 individual VOIP lines as well as hosting an old fashioned PTSN line. The actual device is about £65 plus handsets on top. I use a mixture of Gigaset handsets with mine ranging from circa £30 to £70 each. This IMHO is much easier to configure.

I'm sure there will be others more knowledgeable than, around shortly with alternative suggestions - especially on the ATA front.

It is an odd router if it only has one Ethernet port on it !

What router is it?

Linksys
https://www.gigaclear.com/technical
WHW0301GC-UK
Has one port in - one port out

In reply to a post by farrinaf:

Its not clear from your post as to what the "free ethernet sockets in the wall plate" are - I take it these are something to do with the service provision from Gigaclear. I presume this will be before the WAN side of your router.

The box that Gigaclear provided me with has a fibre input, 4 ethernet outputs and Wifi. It was originally a router but was changed to bridge mode a few months ago. I now can't access the web interface, and there is no wifi. I have a router which is connecte to one of the ethernet ports and which uses DHCP to get its public IP address. I suspect that the OPs wall plate is similar to my box.

In reply to a post by moretvicar:

Do I need one of these
TP-Link TL-SG1005D, 5 Port Gigabit Ethernet Network Switch

https://www.amazon.co.uk/gp/product/B00ZOOJXEG/ref=p...

That would work, but if you want TP-Link this has a metal case and a smaller footprint area; https://www.amazon.co.uk/TP-Link-LS105G-Desktop-Wall...

My own preference would be Netgear - which runs slightly cooler and has a three year warranty. It also has higher quality screened ethernet sockets; https://www.amazon.co.uk/NETGEAR-Ethernet-Unmanaged-...