When you type in a correct/valid number, it'll show you the rates beneath the number [not sure if it was added recently].
I just tested this and I can confirm that you are right, but rates are a bit higher than I expected for calls in Europe, maybe because server and everything else is based in United States.
This is actually the way my old service worked, and I think this is OK as long as it is clearly stated. Either that, or I could add meta mask support, which will give a more anonymous signup. I have thought about both of these options, which one do you think I should prioritise?
I don't want to use anything like this that is connected with metamask wallet, so temporary sign-up would be just fine.
Maybe you can create something like Chipmixer website is using with their Session Token that is used to restore last session.
I don't have anything against normal sign-up registration as long as I don't have to use email addresses or real phone numbers.