Hi, my name is ***** ***** I will answer your question today.
There is a range of Gigaset phones, most of the ranges come in a twin or trio, a list of models which support bluethooth can be found here.
Looking at the reviews and technical specs I would suggest Gigaset S850HX, as it supports Bluethooth, Audio profiles, available as a Single, Twin, Trio and Quad set, has low transmission power \ radiation which helps reduce interference with hearing aids and comes with a range of features (but lacks an answering phone). The cheapest I could find this model for was £130 and can be ordered here.
Please let me know if you have any other questions.