The registration is only available on Android for now and costs a one-time fee of $2.99. The fee is meant to make it financially infeasible for spammers to create thousands of accounts and ruin Signal’s network.
The payment is currently only offered through Play Services, but more payment methods are planned for the future. This means that users who don’t have Play Services, such as people running GrapheneOS without Sandboxed Google Play, won’t be able to try the feature out just yet.
so now instead of just your phone number, now they have a LOT of other info about you. Makes sense. Privacy!