Extra cost calls to my S7 are forwarded to my S2?

I got my S2 solely for those times I forgot my S7. It works great in getting the calls forwarded to it and in the quality of the calls. I didn't ask if there would be additional charges beyond the $5/mo fee I pay when calls are forwarded from the S7 to my S2. Does anyone know if I will be paying extra every time this happens?