It depends entirely on the developers and the owners of the service you use to call. However, if you use a carrier network, then yes, the logs will be stored on server, even for pre-paid cards. The voice is not recorded, only details about the connection, like: time of call, numbers, call duration, carrier networks, etc. You will have to call Customer Support for the network you use to ask them about logs and other recording aspects related to your conversations.
