I had never used phones to pay, or been with others who had used phones to pay, either.
When I pay by myself I always pay in cash, although I am usually with someone else who wants to pay by credit card (or sometimes by debit card). A signal does not seem needed though since they will just tell them they are ready and then will pay immediately when they have the bill ready. When I am by myself, I will pay in cash and I will try to pay in advance when I can (which has other advantages as well, such as to know the price and if I have the money to pay).
When I pay by myself I always pay in cash, although I am usually with someone else who wants to pay by credit card (or sometimes by debit card). A signal does not seem needed though since they will just tell them they are ready and then will pay immediately when they have the bill ready. When I am by myself, I will pay in cash and I will try to pay in advance when I can (which has other advantages as well, such as to know the price and if I have the money to pay).