The short definition
A virtual phone number is a phone number that is not tied to a specific physical SIM card in your device. It exists in software on carrier or provider infrastructure, and whoever controls that infrastructure decides where its calls and texts are delivered.
That single property — no fixed SIM — is what makes a number rentable by the minute, issuable in a country you have never visited, and disposable after one use.
Three kinds you will encounter
Activation numbers are issued for a single verification and last minutes. You receive one code and the number goes back to the pool. This is what most "receive SMS" services sell, and it is the cheapest option because the number is reused heavily over its lifetime.
Rented numbers are held exclusively for you for days or weeks. Every message sent to them during that window is yours, which makes them appropriate when an account will keep texting you, or when you need the same number to persist.
VoIP or "second line" numbers are full calling numbers attached to an app. They are useful as a real second line, but many platforms specifically detect and refuse VoIP ranges during sign-up, which is why a second-line app is often the worst choice for verification despite looking like the most legitimate one.
Virtual number vs eSIM — a common mix-up
An eSIM is a digital SIM profile that connects your own device to a mobile network, normally for data while travelling. A virtual number is a number you receive messages on without a device or SIM at all.
They solve different problems and are often bought together for the same trip: an eSIM so your phone has data abroad, a virtual number so you can verify a local service you have no local SIM for. Neither replaces the other.
Where the limits are
A virtual number will not make an account permanently recoverable, because you do not keep the number forever. It will not defeat a platform that has decided to distrust an entire number range — no provider can promise universal acceptance, and any that does is guessing. And it does not make you anonymous: your payment method, IP address and account behaviour are all still visible to the platform you signed up to.
Understood as a tool for passing a one-time code without exposing your personal number, it does that job well. Understood as a permanent identity, it will disappoint.
When a virtual number is the right tool
It fits when you need a second account and have only one SIM; when a service is unavailable to numbers from your own country; when you would rather not give a marketplace, dating app or one-off sign-up your real number; and when you want a number for a short project without buying a SIM for it.
It does not fit for banking, government identity, or the recovery number on an account you intend to keep for years. For those, use a number you own permanently.