iPhone 3GS

http://support.apple.com/kb/TA38608?viewlocale=en_US&locale=en_US

You can make conference calls on your iPhone by

1. Maknig a regular phone call

2. Tapping the "Add Call" button and making your second call while the first call is put on hold (automatically)

3. Selecting "Merge Calls"

This is true for up to 5 simultaneous calls.