If you can lay your hands on an SDS+ drill, start by making holes for Plasplugs. On an irregular wall with thick lime plaster, you need to go at least an inch into the wall (excluding the thickness of plaster) so try three or four inches. You can get extra-long plasplugs (with screws) sold as frame Fixings. The size of masonry drill you need will be on the plugs. SDS+ drills have a grooved end, not round like ordinary bits, and are very robust. One labelled for concrete will be OK. It will not fit an ordinary electric drill. It is bashed very hard.
You will feel it sinking in like pushing a pencil into cheese. If the drill hits something hard and stops penetrating for more than half a second, take it out and try a new hole two inches or more away. If the hole position is uncertain, fix a wooden batten to the wall to suit the holes, and fix your telephone to that.
For a telephone, try two holes and look for a Keyhole Mirror Plate that you can put on the backing board, and hang over the screwheads.
It is likely to be quite a ragged hole, as the substrate is difficult. Verify that the screw and the plug will both go all the way into the hole, and recess slightly below the surface. Clean out all dust and loose material with a hoover hose or a garden sprayer. There will be a lot.
Buy a tube and metal applicator gun of solvent-free builders grab adhesive (like No-More-Nails, but a cheap own-brand will do). It comes with a pointed plastic nozzle. Poke the nozzle deep into the hole and squeeze the trigger to inject the adhesive paste deep into the hole. Withdraw the nozzle as it fills. Poke the plug all the way into the hole and wipe away excess adhesive with your finger. The next day, it will have set, and will hold the plug firmly and it will not twist, expand, come loose, or crumble the hole.