If your phone is found by someone who wants to return it, they have one problem: no way to reach you. The screen is locked, your contacts are behind it, and there is nothing on display that helps.
Putting a contact line on the lock screen fixes that in a minute, and there are two ways to do it. They are not the same, and most people only know about one.
The two approaches
A wallpaper with the text in it. You choose the wording, the typography and the position, and it is there permanently, whether or not you have done anything else. It is visible from the moment the phone is found.
Lost Mode. Part of Find My. You activate it remotely after the phone goes missing, and it locks the device and displays a message and a phone number of your choosing on the screen. It also stops Apple Pay and lets you track the phone.
Lost Mode is the more capable of the two and it only exists after you notice. The wallpaper is less capable and it is already there. Most people should have both.
What to put on the wallpaper
Keep it to one line, and keep it factual.
A second phone number is the most useful thing, since it works for someone with no internet and no account. Obviously not the number of the phone itself.
An email address is the alternative if you would rather not display a number. Slower, and less likely to be acted on by someone standing in a station.
A first name helps more than it looks like it should. A phone that says "Sam's phone, call 07xxx" reads as belonging to a person rather than as an object, and people behave differently towards the two.
What to leave off: your home address, your full name and address together, and anything about where you work. The wallpaper is visible to anyone who picks the phone up, including people who are not trying to return it.
Where to put it
The lower third, above the flashlight and camera buttons, is the right place for a contact line.
It stays clear of the clock, it does not compete with notifications, and it is where someone's eye goes after they have registered that the phone is locked. Directly under the clock is more visible and it is also where widgets sit, so you would be giving one up.
Keep the type small and legible rather than large. This is information for one specific moment, not a message for everyone.
Set up Lost Mode before you need it
The wallpaper line is the fallback. Lost Mode is the tool, and it needs three things to be already true: Find My switched on, Send Last Location enabled so the phone reports its position before the battery dies, and you knowing how to reach it from another device. Check all three now, under Settings, your name, Find My. The whole thing takes under a minute and it is the difference between a lost phone you can act on and one you can only hope about. If you have Stolen Device Protection on as well, turning off Lost Mode requires your face rather than your passcode, which is the point of it.
The other route nobody uses
There is a third place to put contact details, and it is the one designed for exactly this: the Medical ID.
It lives in the Health app under your profile, and its contents are reachable from the locked screen through the Emergency option on the passcode keypad, without unlocking anything. It was built for paramedics and it works just as well for a stranger holding your phone.
Fill in the emergency contacts section in particular. Someone who finds a phone and taps Emergency, then Medical ID, gets a name and a number they can call directly from the locked device.
Two things to check. Show When Locked has to be switched on, or none of it is reachable. And the emergency contacts have to be actual contacts in your address book, since the feature links to entries rather than storing numbers.
It is not a substitute for the wallpaper line, because most people will not think to look there. It is a good addition, because the people who do think to look are precisely the ones trying to help.
Using Lost Mode when it happens
From another device, or at iCloud.com, open Find My, select the phone, and turn on Mark As Lost.
You enter a phone number and a short message, and both appear on the locked screen. The phone locks immediately, Apple Pay is suspended, and you can see its location if it is online or its last known location if it is not.
You can also play a sound, which is the right first step if the phone might simply be in the house.
Do not erase the phone as a first move. Erasing signs the device out, which ends Lost Mode and removes the message and number that were helping an honest finder return it. Erase when you have concluded it is not coming back.
Quick answers
Can I have both a contact wallpaper and Lost Mode?
Yes. Lost Mode covers the wallpaper with its own message while it is active, and the wallpaper returns when you turn it off.
Will a phone number on the lock screen attract unwanted calls?
It is a risk people worry about more than it happens. If it concerns you, use an email address, or a secondary number rather than your main one.
Does the text stop the phone being stolen?
No. Nothing on a screen does. This is about the case where the phone is lost and someone wants to give it back, which is a genuinely common outcome and the one worth preparing for.
What if the phone is off or out of battery?
Recent iPhones can be located for a period after they are powered down, and Send Last Location reports the position before the battery dies. The wallpaper line still works, since it is visible on the screen when the phone is turned back on.
Where to get it
DON'T Touch Phone Wallpapers includes contact-line layouts with space left for your own details in the lower third, previewed in a phone-shaped frame so you can see the spacing before saving.
Turn on Find My and Send Last Location first. The wallpaper is the second step, not the first.