It is necessary to check that the wire of terminal 6 of indoor station is directly connected to terminal -/~ (minus/alternating) of the outdoor station where, as per diagram supplied in the manual, two wires must be present: one coming from 0~ (alternating zero) of the transformer and the other from terminal 6 of the indoor station. In fact, if the wire of terminal 6 of the indoor station is wrongly connected to the transformer alternating current zero, a buzz in the handset of the indoor station may be heard.