Hallo Parviz,

you forgot to give any specification. Possible amount of compression is defined by your needs.
Voice can be compressed by low-pass-filtering. 'Telephone quality' is achieved by high order lowpass filter with corner frequency of about 3..4kHz. When you have signal digitized with an 8-bitADC, you still need to transmit about 24kbit/sec.
So, that soultion is not adequate.
You are not the first to solve this problem. And many, many developers have searched for a methode of drastically reducing of data. Adaptive delta modulation is one breakthrough. Many years ago, there were good results achieved with less than 16kbit/sec, and today much lower datarates can be achieved, while showing very good results. You have got some links, already by other forum members.

Bye,
Kai