Real signal gen
I have tested this with MC145436 DTMF decoder and LM358 used for condenser microphone preamp. If you are using an electronic hardware decoder this is the DTMF keypad you want. Most are playing back recorded samples, whereas this one is using low level audio framework to generate signal. The iPhone speaker begins to clap out at about half volume for DTMF. Works most reliably for me at about half volume for iPhone5.