A virtual phone number is a secondary telephone number associated with a certain telephone unit, handset, or subscriber. It is not directly associated with a telephone line and is not the actual telephone number but rather one that is used for forwarding calls to a pre-set number. As a result, a subscriber does not need to purchase additional hardware, phones and lines in order to have multiple virtual telephone numbers.
Virtual phone numbers are also known as follow-me numbers in the U.S., or personal numbers in the UK.
Techopedia explains Virtual Phone Number
Virtual phone numbers are set outside of the physical area codes used for regular telephony and they can be configured to forward calls to different specific numbers depending on the time and day of the week. The virtual number can be set to any area code so that people calling from that area will not have to pay long-distance charges. For example, suppose that a subscriber from New York chooses a virtual phone number with a San Francisco area code. Anyone who calls that subscriber from San Francisco through that number is considered to have made a local call. The subscriber only needs to pay a monthly subscription fee for the number.
Virtual phone numbers are especially popular in call centers, which have contact numbers that appear to be in one country, while the call centers themselves are located in different countries and time zones, allowing these call centers to deliver 24/7 service.