The problem with swapping numbers
Most messaging is built on the phone number: it is the account, the address and the identity all at once. That works well among people you know and badly with everybody else, because a number cannot be un-given. Once somebody has it they can call you, message you, look you up on other services that key off it, and keep it after the reason for having it has passed. Blocking is a thing you have to do afterwards, one contact at a time.
The alternative is to make the conversation itself the contact detail. You send a link, you talk in the room, and when it is over there is no number to be held by anybody. Nothing has to be blocked, because nothing was exchanged.
Situations where this is the sensible move
Selling or buying something. Arranging a collection, a viewing or a handover with somebody you found on a listing site. You need to talk while it happens and not afterwards.
Working with a client or a contractor. A room for the duration of the job keeps it out of your personal messages and gives them a way to reach you that ends when the work does.
A landlord, a tenant, a neighbour, a repair. Practical arrangements between people who are not friends and should not have to become contacts.
Somebody you have just met online. Moving off a platform usually means giving out a number. This is a way to move the conversation somewhere quieter without that step, and to walk away cleanly if it goes badly.
A group that is not really a group. Six people who need to coordinate one thing, where a WhatsApp group would mean all six seeing each other's numbers and the group outliving the thing by years.
How it works
Pick an address: klaklak.com/blue-sofa-collection, or anything else nobody would guess. Send that address instead of your number, in whatever channel you are already talking in. The other person opens it, types a nickname of up to eight characters, and you are talking. There is nothing to install on either side and neither of you is asked for a phone number, an email address or a password at any point.
When you are finished you both stop opening the link. After a day with nobody in it the room clears itself and the conversation is gone. There is no contact left in anybody's phone and nothing to unfriend.
Being clear about what this does not do
It hides your number, not you. The other person still reads everything you type, so anything identifying that you put into a message is out there in the ordinary way. A room with a disposable address is not a disposable identity.
It is unlisted, not secret. Anybody with the address can open the room, which is why the name matters: something long and specific rather than a short obvious word. The private rooms page goes through exactly where that boundary sits.
It is not end to end encrypted. Your connection is encrypted in transit, but messages reach our server in a readable form so that the room can display them. For dealing with abuse we log the time, the IP address and the nickname beside each message, which is set out in the privacy policy.
And the usual caution about strangers applies unchanged. Not being able to phone you is not the same as being trustworthy, and a chat room does not make somebody safe to send money to.
If the conversation goes wrong
You can ignore any nickname with one tap and stop seeing that person entirely. If you opened the room you can also delete messages, remove somebody and ban them from returning. And the option that does not exist with a phone number is always available: close the tab and never open that link again. There is no account for them to find, no profile to look at and no way to reach you anywhere else, because you never gave them anywhere else.
Questions people ask
Do you ever ask for a phone number?
No, at any point. There is no field for a phone number anywhere on the site, no verification message and no way for anybody in a room to see one. The same goes for an email address.
Do I have to give an email address instead?
No. There is no signup at all, so there is nothing to verify. A nickname of up to eight characters is the only thing you type before you can start talking.
How do I stop somebody from contacting me?
Close the tab and stop opening that link. There is no account to find you through and nothing of yours in their phone. Inside the room you can also ignore a nickname with one tap, and if you opened the room you can remove and ban them.
Can they find me later through the site?
No. There is no profile, no search for people, no friends list and no account. Once you stop opening the link there is nothing connecting you to that conversation.
Is it safe to use for arranging a sale or a collection?
For keeping your number private it does the job. It does nothing to make the other person trustworthy: the usual care about payments, meeting places and things that sound too good applies exactly as it would anywhere else.
Can I use it to chat without WhatsApp entirely?
For a specific conversation, yes, and that is what it is for. For staying in daily touch with people you know it is a poor substitute, because there are no notifications, no history and no way to reach somebody who is not currently in the room.