You can have a conference call with up to 5 different phone lines involved from your iPhone 3G. Here’s how to bring everybody together onto one line.

1. Make your first call.

2. Tap Add Call and call another line. The first call is put on hold while you are doing this. If you want, you can talk on the second line privately before merging the calls.

3. Tap Merge Calls to bring the current line into the conference with the other caller(s).

Perform steps 2 and 3 to add more calls to the conference.

If you have an incoming call that you would like to merge into the conference, tap Hold Call + Answer, then tap Merge Calls.

Source: iPhone Users Guide