Salam,
Take care that you can't send DTMF on mobile lines, that is to say if you played a DTMF at the mic of one side of the call and received it from the other side it will not be interpreted as a DTMF. as throwaway18 said the DTMF signals are sent on the mobile network as digital signals and the mobile that receives the tone regenrates the tone for human interface or may be a machine that will receive that tone. if you are willing to send DTMF tones on the GSM line so you might connect your mcu to the mobile and send AT commands to it.
you can use the command "AT+VTS="
for more information you can look in the following link.
salam
shereef