@gerardb63 it's called call waiting, if you are on a call and you get the tone that another call is coming in (providing you have the call waiting feature on your line and enabled on your phone) just look at your screen and you should see the option to answer or ignore.
The wording and looks will vary from phone and the OS the phone is using.
Edit: I should have asked is it for cellphone or home phone?
for home phone again it would depend on the type of phone you are using, usually you just press the answer button again to answer the other line so phones may have a dedicated call waiting button.
