Silent Circle is not a carrier and does not provide cellular services. Silent Circle offers a VOIP application for encrypted communications which require an internet connection.
Blackphone2 is sold unlocked and will work with any compatible GSM provider.
It includes a pre-paid subscription for certain services which come on the phone. These are not the same as service plans from a carrier, which are one method to get mobile data access when you're out and about.
They also provide minutes for traditional (unsecure) calls, and allocations of SMS messages. Any carrier plan will be an additional expense and is the same as the plan you have with your current phone.