Like any phone, analog or non-analog, it can only have one connected call at any time, but it can have one call connected and several calls on hold. For a conference call you would typically make a call, put it on hold when answered, repeat for all the required parties and then make a final call that is the IP Office short code to conference yourself and all the held calls.
Doing that manually on an analog device means you need to know how to hold a call and get back to dial tone (the R or Recall or Hold key - the marking varies) and the IP Office short codes for disconnecting the last connected call (needed when the last call you made and held was one that had been answered by voicemail - you don't want that as part of the conference) and for conferencing the calls.
The Knoftel devices (if setup correctly with the IP Office short codes) can automate a lot of that. But even so its still one call at a time, hold call and then conference held calls.
Having an app like one-X Portal for IP Office or Phone Manager makes doing all that a lot easier as they can control the calls and context sensitive controls for adding and dropping calls.