Twilio is one of the most popular ways for businesses who want to use an API to make their own phone communication systems to work with their websites. If you want to use customized phone apps on your site or other online app, Twilio provides a way for you do to that for voip services.
Of course, Twilio does not come without a cost. It is known to be relatively expensive. There are many alternatives that have many of the same features as Twilio, but come at a lower price tag.