If your mobile phone supports a Call Waiting service, you can receive the second call during your current conversation.
Activation: *43#
To cancel: *#43#
To check: #43#

If someone is trying to call you while you are already having a phone conversation, you hear low tones and can swap to the second subscriber, talk to him, and then, pressing a single button, return to the previous conversation. Thus, you can swap between several calls or ignore new calls, continuing your current conversation.
If you wish to answer an incoming call immediately:
Press
If you decline an incoming call:
Press 0
If you wish to hold on a subscriber you are currently talking to and answer an incoming call:
Press 2
If you wish to finish a conversation and swap to the first subscriber you are holding on:
Press 1
If you wish to swap between two calls:
Press 2
You can also use a Call Waiting feature to make a call during your current conversation with another subscriber:
Dial: phone number
Later on you are able to hold on a call or to swap between two subscribers, as it is described above.