" 1) Is programming a device using a telephone set (i.e. an external device) involves learning flash memory ...is it flash memory or something esle that i need to learn?"

It may be flash, but it may be just some static memory that has its contentes preserveds by an external battery/capacitor

" 2) while going through its parts i found that the manufacturer has used a 20 Pins 89c2051 controller on which 24PC, 9943 is also written ... Is using 20 Pins controller is more efficient , economical or size is the factor , what you think..."

Look at the datasheet of the device and see if has the same programming characteristics of a regular 8051... about the size and economical issues... what do YOU think? The device is simple, probably doesn't need all the ports of a 8051... think...

About the books... look for some book about basic electronics (analog and digital) and a good book about 8051, with exercises...

Seems that your main problem is that you want to skip steps in your understanting of electronics/processors... skipping these steps won't make you learn faster or better... will slow you down and make from you a bad programmer/engineer...

All the exercises that you could find in a good book (the simple ones: blinking leds, reading push-buttons, interrupts...) and you skipped are going to haunt you in the future... like when you start to code a clock, with great pain in the proccess and when you finish it you realize that you can't adjust because you didn't learnt interruptions or how to read a push-button.