A virtual number is a telephone number without a directly-associated telephone line, usually programmed to forward incoming calls to one of the pre-set telephone numbers chosen by the client; either fixed, mobile or VoIP.
K4Health
Method to carry private communications network traffic over the public Internet using tunneling or port forwarding which is the transmission of private data over public lines in an encapsulated form.
American Telemedicine Association
Voice over Internet Protocol (VoIP) is a technology that allows to make voice calls using a broadband Internet connection instead of a regular (or analog) phone line.
ITU (International Telecommunication Union)