i dont think u will find any phone that provide anonymity. not because of the apps but because of the phone operating system itself.
Android OS? made by google so u can imagine …(even if u gonna say its open source , there is no company claimed to give u anonymity. security maybe but not anonymity. for example:- blackphone?)
Sailfish OS ? used to be on jolla phones = not much different than google but its better for sure. anonymity ? i dont think so. read their policies.
IOS ? made by apple so u can imagine …(closed source = no way of anonymity)
Windows Phone? made by microsoft so u can imagine …(closed source = no way of anonymity)
QNX OS? made by BlackBerry…(closed source = no way of anonymity)
Tizen OS? made by tizen in association with samsung &intel …(all the companies mentioned here have no relation with user privacy/anonymity. and the SDK (Software development kit) of the OS is closed source = no way of anonymity)
Ubuntu Touch OS? made by Canonical Ltd … (Canonical feed on your information , u can read articles about ubuntu and privacy. canonical = microsoft)
regarding the app itself , i heard about it but didnt read much about its company so i dunno regarding the app safety. but it has similar features like TOX chat. which i have read much about and i really advice users to use it. (dont relay on it for high private conversation because the app still under heavy development).
also dont forget to use on ur phone = Orbot +Orwall (orbot firewall).
so u can be anonymous with ur apps but not with ur phone OS.