For each call in you need a free line in. To get more than one extension to receive these calls you can set call groups up to ring the extensions answerable to that lines calls, as waseem states above
e.i. Line 1 rings call group 701, extensions that you have set in that call group will ring all together or with a delay that you set.
Line 2 rings call group 70, extensions that you have set in that call group will ring all together or with a delay that you set. Extensions can be in more than 1 group
Bottom line is that you can only receive calls if you have broadvoice voip lines that are not already on calls