the one base is connected via the green socket and works to make a phone call but now not sure which phone we need to buy that will work in a meaningful way
I can only guess that mean you have a system with four cordless handsets, one of which charges on the base? If that is the case then as already said, nothing has changed except possibly the distance to the base if the handsets are now further away than they were.
If you have something different then you're going to have to be specific with exactly what.
Base station ring volume/tone is generally set from one of the handsets connected to the base.
@arturo-roger as has been repeatedly stated, you don't need to buy anything.
thanks for reminding me