Yes, this calculation of .5mb/minute on WA call is correct.
in case of RJio, calls on their VOLTE technology will not be charged for data. this is true for volte supporting phone or non-volte phone but using over riding app RJio Join which ultimately uses VOLTE technology. both above does not require even data connection to be active on user’s phone.
Phones using RJio Chat for call will be charged for data on both the legs. receiving as well as calling side. because they are pure data call and hence like WhatsApp call.
Video Calls on Volte will be charged for data in any case.