Hi Narin,

Using MT8888/8880 is tricky, but only untill you have done it. After which you would fall in love with this chip. The only problem is that it monitors call progress by detecting the ring / busy tones which are audible on the line after you have dialed. I don't like this. There should have been some other signal based caution from exchange for call accepted or busy, like you get signal for CLI.....

Anyway, now your question:

Looks like you are trying to transmit/ receive data over DTMF. Making a DTMF modem......

Please confirm what is your design obejective ??

Simon