We use a lot of SNOM 300 VOIP phones at our sites, and have just cracked a way to make conference calls on them, which is not obvious or covered by the manual. If you are trying to do this:

1. Login to your phone using the web GUI via http://its.IP.x.x and select function L4 – directory, (or any function you dont use)
2. change the function tag to   F_CONFERENCE
3. save

to make a conference call,

1. call the first call, speak and put on hold by pressing the L1 line 1 button
2. dial the second number and speak to them
3. press L4 and all 3 callers are connected