Please recommend a no-sim messenger

Anything that uses XMPP.

What is the difference between XMPP vs matrix?
- Using this very good comparison site in German as base and ignoring the similarities:
  
  Technical
  
  XMPP allows for various different client styles while Matrix doesn’t
  
  Fully featured, mature text-only clients for XMPP exist, but for Matrix there are only beta text clients available
  
  Matrix handles/addresses are always public in chat rooms while XMPP chat rooms can be configured to hide them
  
  Matrix has multi-server chatrooms for reliability in case of server or network issues while XMPP chatrooms alre always bound to one single server
  
  Matrix Chatrooms can be encrypted. XMPP chatrooms can be encrypted or not encrypted. (there’s no further explanation on this point, I assume it’s meant that Matrix servers can turn on encryption and that’s it, while XMPP servers allow the chatroom administrator to decice and do not force one or another)
  
  Administrative data is stored on one XMPP server. Matrix administrative data is stored on the servers of all connected users
  
  XMPP is modular and the protocol itself can be extended while Matrix is monolithic
  
  XMPP protocol uses XML while Matrix protocol uses JSON
  
  Matrix focuses on reliability and availability of chatrooms, XMPP focuses on features and extensibility
  
  XMPP uses less system resources than Matrix
  
  Chatroom data storage is done only on the XMPP server the chatroom is running on, while Matrix stores chatroom data on all of the servers of the connected users
  
  XMPP directly sends a message if the connection is open, otherwise a push notification is sent. Matrix only sends a push notification to the client and the client has to pull the message from the server
  
  Organizational
  
  XMPP is an IETF standard while Matrix isn’t
  
  XMPP board and council are equally elected by all members. Matrix is a “single-party system” where the board decides who is allowed in the board.
  
  For XMPP all members are allowed to question/check/validate the board and council and there are annual elections. For Matrix, the Matrix.org Foundation (technical council) and New Vector Ltd. (service provider) expect trust from the community.
  
  On the XMPP board and council, all actors have to name their interests and their employer (this is to prevent having more than 15% of board/council members from the same company which would give a single company too much power). For Matrix there is no known information about such a clause.

it depends on who you want to talk to. If the protocol does not matter I suggest matrix. Otherwise I suggest more specifics.

Just my friend who doesn't have a sim card, he has an android phone
- Matrix / Element should work fine then.
- Signal is a very popular option.

SimpleX Messenger is a great option.

There are quite a few. In order of preference, I used/tried:

XMPP (Android-Conversations; Windows-Gajim)
Element/Matrix
Session
Discord
Slack
Twinme
Teams
Skype
Facebook Messenger
Instagram DMs

There's tons of free options for XMPP clients and servers.

Is it better than matrix?
- On user side matrix has more features (or maybe more bloat, element is kinda huge), but XMPP servers are easier to self-host
Won’t burn thru your battery or server storage either

If you want a simpler solution than Matrix and don't mind using a non-opensource app you could try twinme. You only need a username and a picture to create an account (could be anything, "Jane Doe" and a white square if you want). It has a couple unique features like multiple identities and click-to-call links, which you can share with people who don't have the app so they can call you using only a web browser (e.g. on craigslist, lost pet flyers, ...).

I work on this app as a full-stack dev, and for what it's worth we only upload/store data that is strictly required for the app to work. We're lobbying to open source it because we have nothing to hide and we know how important it is for the privacy/security minded folks, but it will take some convincing before management agrees to it...

An independent audit would go a long way as well...looks interesting.
- Thanks for checking us out :) We had an audit done by quarkslab a few years back, and another one coming up this year since we're currently implementing verified relations (signed SDPs). I'll edit my comment with a link when I get back to work next week.

Threema. It's e2e encrypted, comes with essentially everything a modern messenger needs and doesn't require you to sign up with your phone numbers.

Signal is probably your best bet for a solid user-friendly (and privacy respecting) experience with calls. Matrix/element is another decent option (its federated like lemmy and mastodon) but a bit less intuitive.

I don't know why people are downvoting you, might have better luck in something like a technology-centered community or putting some more information like what device or uses you have for it.

If you're looking for an app for a mobile device while connected to wifi, there are a lot out there but they are all pretty sketchy in my experience (thinking like text-now which offers a free limited number to use but privacy and ad-blocking get thrown out the window). If you're looking for a messenger that doesn't have a number associated with it, something like matrix that was suggested should work (or whatever preference the people you communicate the most with prefer).

Exactly, I'm looking for an app that only requiere wifi and supports calling. I already tried jami but it's not very reliable for some reason
- hmm, well it's unfortunately usually a "you pay for what you get". If you're not paying then you're gonna end up as the product. I only have experience with "text now" which I've used in a pinch before and it has some limitations on the calls which you'll usually end up running into on any app. I wouldn't be surprised if there was some better ones out there. If you're worried about what kind of App you're downloading you might try the !privacy@lemmy.ml community as someone there might have a better source that isn't constructed to mine your data.

Signal. If you don't want to use a phone number to sign up, then Simplex Chat, but usability is bad.

They don't have a Sim.
Pretty sure you can sign up with a username now for signal. No number required.
- You can hide your number but you still need a SIM