The dial tone tells you that the distant machine [at the telephone exchange] is ready to accept the digits you are about to dial. Thus it is a proceed-to-send [PTS] signal. It is like the erstwhile operators demanding to know the party you want to get connected to when they ask number please. In a central battery system the PTS is also dispatched centrally as a dial tone. When there is no central battery, as for example in the case of a mobile phone, the PTS has to take other forms. Since a screen is readily available one can locally invoke the equivalent of the number please, record the response of the mobile user and complete the call origination on the push of yet another button.

This should raise a supplementary question -- can a PTS be assumed similarly in a wire-line land phone providing there was a screen associated with it? Yes of course, not only will that enable you buffer the digits to send but also exchange SMS or be advised of the caller-ID on an incoming call