Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
DTMF isn't used with ISDN. ISDN relies on bytes which contain the encoded digit. Not many people use ISDN for phone calls these days, by the way
(some countries use it more).
Restricted to phone calls, there is no "advantage" or "disadvantage" of DTMF - there is no other way to get the signal to the phone company if you're
on an analog line.
For non-analog-phone-line use (POTS), then DTMF is archaic, since there are other more efficient encoding methods.
You are mixing apples and oranges. DTMF is used just for the transmission of the "dial" information from a telephone keypad to the central station. ISDN is a standard for digital transmission of voice, data, and video, etc.