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Recovery code

It's a 1 letter + 5 digit code

I had the exact same problem, my solution was to send a ticket to Steam support asking for the number to be removed. It's not as bad as people joke, I had everything sorted out in about 3 days.

It wouldn't be much of a protection if you could, would it? Did you permanently associate your Steam account with a temporary phone number that you either can't remember or can't get access to from your wireless carrier?

I'm in the same situation at the moment. You need to open a support ticket to Steam and ask them to remove your phone number.

I sent them a ticket on Friday, got a response on Monday that they need proof of legitimacy (like a paypal receipt for any purchase or a picture of a box case with a key i activated on the account). Haven't heard back from them since

Settings -> Account -> Manage Steam Guard Account Security. There's an option to remove/ disable Mobile Auth. If you already have the new phone, I don't see any reason you wouldn't be able to remove the old and then just set up the new one from scratch.