Connect ATA to existing phone system

Hi, firstly may I apologise for my lack of technical knowledge. I am a somewhat technophobic senior citizen.
I am shortly having full fibre broadband with digital phone service installed.
I understand that the equipment being installed is an ONT, an EERO 6 router and a Grandstream HT801 ATA. The HT801 has a rj11 output socket.
My problem is that I wish to connect the ATA to my existing internal phone network via the existing master socket, which accepts a flat type plug rather than a square rj11 type plug, so as to be able to use my existing analogue phones in the various rooms throughout my house.
I have searched online and have discovered several different methods i.e. (1) a cable from the rj11 to flat plug which will fit into the master socket in the same way that a phone would. (2) a cable from the rj11 to the A & B terminals in the master socket (3) a cable from the rj11 connected to various terminals in the master socket. I would also be happy to accept any other method which is known to work.
Having looked online I have found a number of cables which may do the job but I am totally confused by the different types e.g. 2 wire or 4 wire, straight through or crossed.
Can anyone help me in this matter (with full details please) in very simple language?
Sorry if I have gone on too long.
Thank you in advance.

Regarding the plugs, there are various adapters around which can be picked up for a few quid, just google for "BT socket to RJ11 plug Adapter". That will sort out the conversion between the two different plug types without having to rewire.

There is also a guide here https://www.thinkbroadband.com/guides/digital-voice-... and a discussion here https://forums.voipo.com/showthread.php/4461-How-to-...

The main issue is the existing connection on the master socket is an output from the BT network with the extension wiring teed off from it. If you were to plug an ATA directly into the existing master socket you would have two power sources conflicting with each other, at best preventing the phones from operating correctly, at worst damaging the ATA or Openreach equipment. Just migrating your phone number to digital voice will not necessarily disconnect power from the external line.

Openreach did experiment with a replacement faceplate which isolated the external line allowing the digital voice output from a router to be safely reinjected into extension wiring, however it was never released. There are a few threads on this forum regarding voice reinjection, also useful articles about making your own https://support.aa.net.uk/VoIP_How_to:_Voice_reinjec...

An alternative would be to use a Gigaset N300 DECT base station which just has to have a wired connection to your router. and Gigaset Dect phones. The base station and the phones all need power. You would need a VOIP ISP such as A&A or Voifone. If you wish to keep the landline number you would need to port it to the VOIP ISP.

Much more expensive but easier to get working.