This option is available for Secure SIM users.
You can use Conference feature to initiate conversation between two subscribers and to be aware of what they are talking about without them knowing that you are listening to them. To do that you do not need either access to their handsets or any Spy programs. Moreover you can set a substitution number for each of the two subscribers.
A Conference Call costs 1 GBP per minute, the fee is deduced from the Master Account of the SIM card. A balance for the Conference Call shall be not less than 5 GBP.