To use two lines during a telephone conversation is possible with the option "call Waiting" is activated via the phone menu. Most mobile operators, this service is free, but just in case, check this point by contacting the customer service of your operator.
The option "call Waiting" is executed via phone menu to call settings, and for most phones will not differ from the following examples. So, Nokia should enter the menu, then select "Options" - "Call" and to activate "call Waiting", and to enable this same option in iPhone, open "Settings", go to "Phone" and enable "call Waiting".
Once the option is enabled, at the time of the telephone conversation you will hear a beep informing you that someone else calls you. Looking at the display, you can activate the incoming call without interrupting the conversation on the current line.