Separate amplified speakers for landline phone

I want to connect the landline phone handset speaker to some amplified speakers. the idea is to splice the speaker output to a switch, then switch between the handset and the amplified speakers via a 3.5mm jack. just like the old Charlies Angels set up in the 70s. The reason is that my mum is hard of hearing now and does not like technology. Likes to keep it simple, to a point I’m thinking of a 70s TV as well. But the modern handsets with speakers still don’t cut it and she can’t use the phone because she cant hear them without the speaker and still struggles with turning the speaker on. This way I can keep it switched on and only turn the speakers off when someone else is using the phone. I can’t find anything then just has a numerical dial pad with a 3.5mm jack coming out the back.