As you mention 240V/50Hz, presumably you are in the UK?. Regulations governing connecting apparatus to the UK phone system are extremely strict - potential penalties include unlimited fines and jail sentences!.
You certainly shouldn't be attempting anything like this unless you know exactly what you are doing, and are fully aware of the possible consequences.
Other countries are much more slack regarding connecting to the phone system - but in the UK no homemade apparatus at all may be connected, unless you send it for approval (which costs thousands of pounds).