Voice Over Internet Protocol.
It's Skype and it's equivalents. Some just have software to put on your PC, others provide handsets/adapters to plug into your router.
I worked abroad for about 10 years and had a Vonage account with a box I could plug into any network router and a mobile for joining any available/free WiFi net. Gave me my own UK local phone number usable from anywhere in the world and also had voice mail.