Why is Google asking for my phone number?

On rare occasions Google may ask you to provide your phone number to regain access to your account.

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I am being asked to provide a phone number to get back into my account, is this legit?

If you logged in and were taken to a special web page that asks for your phone number then yes, this is a legitimate Google site. You will need to provide a phone number in order to continue.

Why does Google ask for this?

It is an anti-spam system. Here's how it works. 

Each phone number can be used to re-activate accounts a small number of times before it won't be accepted anymore, and each Gmail account can send to a limited number of recipients every day. 

Because most people don't buy things from spammers, they need to work with lots of accounts in order to send enough email to make money. If we think an account might be being used by a spammer, we lock it until the owner provides a phone number. So, spammers need to have access to a large quantity of phone numbers to unlock all their accounts. This is very expensive for them but pretty easy for most regular people. In most cases it is expensive enough for the spammer that it becomes unprofitable for them to spam, so they give up.

I am not a spammer. Why am I being required to provide a phone number?

Your account was probably compromised and used to send spam. When we detect this we send the account to phone verification. This is to lock out the spammers until you have the opportunity to sign in to your account and change the password. We have to do this because it may be days or even weeks until you next check your mail, and it's likely spammers will try to repeatedly abuse your account during that time.

What will Google use my phone number for?

We will send you a number that you have to type into the web page. After that it won't be used for anything else. In particular it will never be used for marketing things to you.

I don't want to provide my phone number, that's personal.

You can provide any phone number that has not been "used up". It does not have to be your own. Even payphones can work.

I don't want to provide any phone number at all.

You can follow the instructions on the verification page to contact support. However it may take us some time to get to your ticket, as these cases are lower priority than people who have been genuinely locked out without any way back in. We strongly encourage you to go through the phone verification process - many people have already done it, and we keep the numbers used private.

The code never arrived.

In some parts of the world SMS is not completely reliable. If the code never arrives, try logging in again and using the phone call option instead.