Gil,

"Kindly concentrate on my "question" and not the apparent errors ..."

Say What????

You can validate that the straight line code works, in a simulator. You will find that external interrupt does not require that you reset the interrupt flag.

As has been indicated, watch PSW it's a common source of problem in interrupt processing. Again, a simulator helps here.

Using a 'scope or logic proble, check to validate the DTMF decoder is negating it's interrupt signal when you read out the DTMF nibble during the interrupt routine. Trace signalling may help toggle a port bit after reading the DTMF nibble and compare the strobe negation against your trace signal's edge.

regards,
p