How do "they" know my cell phone number?

Yesterday during the storm, and several weeks ago, also during a storm, we both got alerts on our cell phones.

Ours are old fashioned flip phones, pay as you go (Tracfone) no plans. And only a few friends and family members know the numbers. Is someone calls and leave a message - as happened once with a doctor's office - it could be a couple of days before we find it.

So how to "they" - the National Weather Service, also an Amber alert - know our numbers? Do the network, Tracfone in our case, provide then?

And last night was really something.