THE TIME OF RECORD IS 4 HOUR.
-->go with AT89C51SND1. so,here you can use your hard disk(of computer)with lot's of memory.

HOW CAN I DETERMIN DATE AND TIME OF MESSAGE?
-->you can use RTC(real time clock).so make a small code for internal software. update it when clock is there and return in your rutin(interrupt base).
when phone comes take the reading of internal clock where you update it(from used location).so,you can have record of it.

CALLER ID
-->for caller id DTMF(dual tone multipal frequncy)detection is used..
link :http://chaokhun.kmitl.ac.th/~kswichit...f/dtmf.htm

(this link will get you a clear picture)
Mihir Patel.