Threema

I guess it depends on where you are. It is available in Australia, that I can tell.

Whether it is paid placement I don't know but there are merits in Reece's recommendations (from point of view of a retired IT guy).

1) It is non-US, so not subject to cloud act, which only applies to US based corporations. Cloud act allows US law enforcement (and some other authorities) to access any information related to private communications available to those corporations even those stored overseas, especially those on cloud, via warrant or subpoena, or if you are NSA just feel free anytime you feel like it. So you think you are a Whatapps user in Europe and US authorities can not access text messages on your phone? Think again.

2) It is anonymous. It does not require phone number or email address. But that doesn't mean nobody can identify you, since it is a paid app they should be able to trace the payment you made through credit card or bank account information. It is not like you can pay through crypto. Threema has your account information and payment information, so you are not anonymous to them. If you pay through Google play or Apple store then you can't really hide from NSA, since both are NSA data custodians. Also if the app is on your phone then they know it is you. Like Reece said you need to have a burner. You think that is it, no, not enough. If the burner can be opened with your bio-metrics, then you can't really deny it is yours. The burner could also have traceable origins they maybe able to find out where it is bought, and there could be cameras in that place, or you paid with a card in your name.

3) Nothing is stored on cloud. But be careful, that is their claim, for all we know this private Swiss company is owned by NSA. Also everyone still needs to connect to a sever in Switzerland, while it might not store text messages and conversations, but there are going to be meta data of communications, like when they were started & ended and who you have been talking to. Even the text messages are not stored on cloud it is still on your phone, so unless you delete them every time, it is still not safe. For example they compromised your phone through WhatsApp then they can read everything on your phone, including your Threema messages.

So if you want complete privacy it is very difficult these days. Unless you are extremely security conscious, most of us just don't bother.

PS: Reece is also right about WhatApp, just don't use it unless you have no choice.