Olvid is a messaging service from France which is useable without a phone number and accessible under Android and iOS (which allows running it on a tablet without a SIM-Card). It provides an end-to-end encryption which includes metadata and thereby can assure anonymity of its users.
For the private person, Olvid is free. For businesses, it offers the business plan at €3.99 monthly per user with 10 users minimum, which enables some additional features, like initiating secure audio calls.
Read more on the pricing page of the service.