Before you wipe or sell your old phone
As long as your old phone stays on and keeps your accounts signed in, it's your plan B: the trusted device you could use to approve a sign-in or recover something. The moment you wipe it, that plan B disappears.
In short: before you factory-reset or sell your old phone, confirm the new one already has everything it needs (codes, passkeys, linked apps). And before you hand it to someone else, remove your account: if you don't, the phone's own anti-theft protection can leave it locked for the new owner.
What happens
Your old phone doesn't stop being useful the moment you activate the new one. As long as it keeps a session open in your email, your password manager or your authenticator app, it's still a trusted device: something that isn't "the phone you just switched from," and that could help you if the new one runs into trouble in the first few days.
On the other hand, both Apple and Google protect phones against theft with a mechanism that locks the device if it's factory-reset without permission: on iPhone it's Activation Lock (tied to Find My), and on Android it's Factory Reset Protection. Both ask for the account that was on the phone before to be able to use it again after a reset. This protects against theft, but it also works against you if you sell or give away the phone without removing your account first: whoever receives it will find a locked phone asking for your credentials.
Why it happens
Because these locks are designed so a thief can't wipe the phone and resell it as if nothing happened. The mechanism doesn't distinguish between a theft and a legitimate sale: it only knows the device had an account and that someone has reset it. You make the difference, by removing the account before you wipe it.
It's the same logic, mirrored, as why the new phone asks for so many verifications: the protection that costs you a bit of patience signing into your own accounts is the same thing that makes a stolen phone worthless to whoever stole it.
Who it hits hardest
People who sell or give away the phone the same day the new one arrives, without waiting to confirm everything works on the new one. And people who factory-reset the old phone to "leave it clean" without first removing their Google account or signing out of Apple, then hand it to someone else locked without realizing it.
It also hits people who hand the old phone to a trade-in or recycling service: many of these services warn that you need to remove the account first, but they don't always check, and the phone can end up locked in their warehouse instead of reaching another user.
What to check before you switch
- Have you confirmed the new phone already has your authenticator codes and your passkeys?
- Are you going to keep the old phone around for a few days as a trusted device before wiping it?
- Do you know which account (Google or Apple ID) you need to remove from the old phone before selling or giving it away?
- Have you decided what you're doing with the old phone's physical SIM or eSIM?
If it already happened to you
If you already wiped the old phone without removing your account and want to sell or give it away, you'll need to sign back in with that account to finish setup and then remove it properly; if you don't remember it, the route is Apple's or Google's official account recovery, never a third-party method promising to "free" the phone. If someone handed you a phone locked by the previous owner, ask them to sign in and release it: there's no other legitimate way.
Be suspicious of any ad or service offering to "unlock" a phone with Activation Lock or Factory Reset Protection without going through the original account: at best it doesn't work, and at worst it's a sign the phone you're being sold is stolen.
Official sources: Apple: what to do before you sell, give away or trade in an iPhone or iPad · Google: prevent others from using your device without permission