Is the cost of a secure phone call different from that of a standard out?

PrivateGSM VoIP:  it works using the Internet on your mobile. It can be very cheap or vey expensive, depending on conditions. If you use a WiFi, tou won't have any additional cost (and much better quality). If you use 2G/3G mobile, you will be charged according to the contract provided by your mobile operator.

PrivateGSM CSD: Generally not, but rates may vary depending on the mobile telephony operator. Please check the cost of CSD data line phone calls with your mobile telephony operator customer service.