I like Signal, but I am not sure if it is 100% decentralised. The privacy level is high as compared to other messaging platforms. I don't know if they use phones as nodes because I do not understand the technical side of it. If you have not used it before, try it out and see if it meet your criteria.
I've heard about Signal and that's thanks to Elon's tweet about them. But are they a blockchain app? and its description says that it's not decentralized but it's privacy is higher than the usual centralized messaging app that we used to have.
I don't think it's the one that you're looking for OP. It needs internet to operate as per description.
Signal is a cross-platform centralized encrypted messaging service developed by the Signal Technology Foundation and Signal Messenger LLC. It uses the Internet to send one-to-one and group messages, which can include files, voice notes, images and videos.