The hardest part of a strange text is rarely the text. It is knowing what, if anything, to write back, and the pressure to answer quickly makes everyone worse at it. So this page is mostly wording you can copy, with a note beside each piece saying when to use it and when to leave it alone. First, a short explanation of why the number that texts you is so often not the number that calls you.
Why the two numbers differ
Texting and calling are often run by different systems. A company may place calls from a customer-service line and send appointment reminders through a messaging service with its own number. A person may call from a mobile and text from an app that gave them another one. Neither is odd, and it is why searching for the calling number sometimes turns up nothing about the text.
Senders come in a few kinds. Ordinary ten-digit numbers belong to individuals, small businesses and local services. Short codes, five or six digits long, carry alerts, codes and bulk messages from larger senders, and you cannot call one back like a normal number. Toll-free numbers (800, 888, 877, 866, 855, 844 and 833) often send business messages that match the company's toll-free line. Odd or very long senders can be email-to-text relays, and those are often spam. The number reader works on ten-digit numbers, so it will not recognize a short code, and an ordinary number's area code tells you only where it began, as with calls that show your own city.
Scripts for texts
The unknown sender with a link
Use it for: a message you did not ask for, with a link and a deadline. The wording is nothing. Do not tap the link, and do not reply. A reply tells the sender your number is active. Forward the message to 7726 (SPAM), which passes it to your carrier, then block the sender in your Messages settings. On an iPhone you can also use Report Junk under the message, where it appears.
The business you recognize and no longer want to hear from
Use it for: a sender you know, such as a shop or a clinic, texting more than you want.
STOP
Reply with that single word. For a known business it is the normal way to opt out. For an unknown sender, do not send it, because it may only confirm that your number works.
The number that used to belong to somebody else
Use it for: texts and calls addressed to a stranger by name. Numbers are recycled, so after a person gives up a line it can be assigned to someone new, as the porting checklist explains.
Hello, this number no longer belongs to the person you are trying to reach. Please remove it from your list.
Say nothing more. Do not confirm any detail about the previous owner and do not offer to pass a message along.
The friend on a new number
Use it for: a text from a "friend" or relative who suddenly needs money or has a new number. Do not answer on the new number. Call the person on the number you already have.
I got a text from a number I don't know that looked like it was from you. Was that you?
If it was not them, you have just saved them a phone call from every contact they have. If it was, the checking cost you a minute.
Scripts for calls
Asking an organization whether it texted you
Use it for: a text that says it is from your bank, a courier or an agency. Do not use any number in the text. Open the official app, or find the number on your card, a statement or the organization's own website.
I received a text from this number about a problem with my account. Can you tell me whether that message came from you, and whether there is anything I need to do?
A real organization will answer this in a few seconds. Real messages from companies tend to name the organization at the start, say what they are about and give a way to stop, often by replying STOP. A delivery notice normally carries a tracking number you can type into the carrier's own site.
When the call and the text come from different numbers
Use it for: a call and a text, both claiming to be the same organization, from numbers that do not match. A mismatch alone is not suspicious, since businesses often use separate systems, but verify both before acting on either.
You called me from one number and texted from another. Can you confirm that both belong to you?
This is a good question to ask on the organization's official line. Do not ask it of the caller who rang you, because they will say yes.
If you already gave a code or paid
Use it for: a moment when you shared a verification code, card details or a payment because of a text. Call your bank at the number on your card.
I replied to a text that claimed to be from you and I gave out a code. Please check my account for activity I did not do, and tell me what to change.
Then file a report at ReportFraud.ftc.gov. One rule I follow without exception: I never read a code aloud to someone who phoned me, because a legitimate company does not ask for it.
Notes on filters and landlines
Carriers and phone makers filter some texts, and the options differ. On an iPhone, Settings > Messages has a Filter Unknown Senders switch, and on Android phones using Google Messages there are spam protection settings in the app's menu. Names change between versions, so look in your own settings. Filters can misfire, so if you are expecting a one-time code or a delivery notice, check the unknown-senders list before assuming it never came.
A text to a landline may fail, because some providers support texting on a landline and some do not. That says nothing wrong about the person, and a call or a mobile number is the better route. For patterns of repeated senders, the call history walkthrough works for texts as well as calls, and for the plus-sign numbers that sometimes text, the essay on country-code prefixes explains why +1 can mean Canada or a Caribbean country as well as the United States.