Using a mobile anonymously with encrypted messages and voice is challenging. With Signal text and voice are encrypted but it sends your contacts to Signal which makes me uncomfortable. With Orfox you can browse the web without revealing your IP address but the GSM module tracks your location.
With a small budget you can however buy a second hand mobile and dedicate it to anonymous communications, as long as you remove its battery when you’re not in a public place. You do not leak your contacts to Signal and the GSM module tracks a SIM card that is not associated with you. Here is a detailed description of the preparation of an anonymous phone I did today. This is not rocket science and I’m sure lots of people already know all of that. But I did not find a HOWTO and it took me some time to figure it out.