Because it is cheaper and the voice quality is often as good

VOIP (Voice Over Internet Protocol) allows a voice conversation to be carried over the same line or connection used for accessing the internet.

Because the transfer of data over the internet is carried out in vast quantities at huge speeds, the "bits" (packets) making up the voice conversations can be carried easily, and since the cost of most internet connections are based upon bandwidth considerations, VOIP is carried for very little additional cost - if any. So to call someone else on the same service as you have is like dialling an extension number in an office, and often costs nothing.

If your call is going to a telephone number on another network or in another country, the VOIP can be carried most of the distance to an internet centre nearest the physical location of the person you are calling, and then conventional telephony (cable or wireless) is used to deliver the call on the final leg of its journey.

The return journey is made in reverse to give you a two way telephone conversation. Because so much of the physical distance covered by the VOIP packets is via the internet (using Internet Protocol), call prices are lower than conventional telephony.