How can I check if a phone number is live?

TL;DR;

You want to know if a phone number is currently registered on a network and ready to receive calls? Carriers do not disclose this information, sorry.
But there is a workaround if you are ready to send an SMS to your users.
Most SMS delivery services provide a delivery confirmation if the handset is on and registered on the network, otherwise the status will be waiting or rejected.
You can even ask for a confirmation from the users themselves using a service like Twilio/verify

In this article, we've detailed what information you can get (or not) about a phone number. We made it clear that the 'registered on network' field is not among what you can get even if willing to pay for it. There is one way, however, that will get you as close as possible to the current state of a phone number. Let's see how it works.

User Validation

Validating phone numbers can be done in many steps, the last of which is actually contacting the number via SMS or voice. While we call this type of service User validation, others call it phone number validation, or any other variation. Names are not important, what's important is how and when to use this type of service and how much should you expect to pay.

How does it work?

User validation consists of sending a code by text message or automated voice call to a number and asking the owner -if a person is there to take the message- to return the code via a web or mobile app. This assumes that the owner has access to a web or mobile app channel and that they are willing to provide this confirmation. Truth is that even if the owner is not willing to return the code or has no mean of doing so, you still get the confirmation that the SMS or voice call was delivered to the handset. Services like Infobip.com will return a delivery status that will say DELIVERED_TO_HANDSET, that is if you are lucky. If the destination handset is not reachable at the moment of delivery for any reason, you might get a WAITING_FOR_DELIVERY status which means that the message was delivered to the destination network but not yet to the handset.

When to use User validation?

If you have a large list of phone numbers to validate, it is hard to imagine sending calls or text messages to thousands and expecting them to answer. Many will not be ready or not willing to answer unknown unexpected calls. Trying this will lead to many false negatives. Real valuable contacts that did not get back to you for different reasons. User validation is more effective when initiated by the users themselves. For example, at signup or as a reaction to an email inviting users to validate their number. Users will be logged to an app where they confirm the number, initiate sending of the code and returning it at the same time. This way the user is expecting the SMS or call, increasing the chances of success. Though, you can still initiate sending SMS messages to large lists, relying on the delivery status to infer the live status of a phone.

How much does it cost?

A service like twilio.com/verify will charge $0.05 per validation in addition to the cost of the SMS or voice call which depends on the countries/carriers to which your numbers belong. Careful with this last part. SMS cost (usually less than voice) can be in the range of $0.005 to $0.5, which can bring the invoice for a large campaign to thousands of dollars.