In an intallation containing at least one stationary unit (5) with a radio transmitter and a ratio receiver, both with short range, and at least one, preferably several portable, wireless telephone sets (4) with a radio transmitter and a radio receiver, both with short range, for exchanging telephone messages with one of the stationary units in the installation (1, 2, 3) the stationary units (5) are each adapted simultaneously to exchange telephone calls with more than one portable telephone set (4) on radio speech channels, adaptively selected among a plurality of radio speech channels common to all radio transmitters (9, 31) and radio recivers (10, 32), there also being included a radio exchange (6) to which the stationary units (5) are connected, this radio exchange being adapted to switch a telephone call with decreasing intelligibility between a portable telephone set (4B) and a first stationary unit (5A) to a telephone call with increasing intelligibility between the same portable telephone set and a second stationary unit (5B). A compressor (20, 41) coacts with each of the radio transmitters (9, 31) for time-compressing the speech signals, and an expander (12, 42) coacts with each of the radio receivers (10, 32) for time-expanding the speech signals.