Welcome to this user forum for BT Retail phone and broadband customers.
What size is the plug on the end of your phone lead, as it should be a standard phone plug like this?
That would be an RJ11 plug. Usually when you buy a phone, an RJ11 to phone plug adapter is in the box, should you need one.
They are easy and cheap to obtain online.
You may be able to get one locally.
Presumably an RJ11 connector.
The phone wouldn't have been supplied like that originally so someone has either changed the cable or removed an adapter like this.
If you no longer have the original then you'll need to buy a replacement RJ11 to BT431A adapter. Just make sure it's a 1:1 connection & not crossover.
