For the past 15+ years we've had no faults with the phone in our small business.
Normally, if a call was answered and a second person tried to ring in, the other phones in our system would ring and could be answered as usual up to the maximum of two calls at once (two lines).
As of late, when a call comes in and is answered, then a second person tries to ring through, the other phones will only ring twice then fall dead.
We're using this ancient machine here which is probably well out of date. Does anyone have any information/solutions to help us out here?

Thanks in advance.