What converts analogue signals to digital signals for transmission over telephone lines?

Answer 1

Actually you have this backwards, the telephone lines can only carry analogue signals (voice band, 20Hz to 3KHz) not digital signals.

The device that converts digital signals to analogue signals for transmission over telephone lines is called a phone MoDem(this stands for phone modulator demodulator). At the opposite end of the line it of course converts the telephone analogue signals back to digital signals.

Answer 2

VoIP is voice over internet protocol. From telephone handset voice is transmitted to voip server then voice is converted into digital signal and transmitted to internet network. Check here for some information about what digital line is: http://www.ringcentral.com/features/digitalline/overview.html

Answer 3

When you speak into your telephone receiver, the audible sound is amplified and then passed through a digitizer, which, as the name implies, digitizes the signal--converts it into numbers.