Depends on the practice set-up.
The one I am familiar with is a large Heme/onc practice. Office hours are basically 9-5, usually with an hour or two gap in between. Each doc has a bunch of their patients admitted to the hospital at any given time, and has to round on them daily, and take their own calls if anything comes up overnight.
Every weekend one doc takes signout from all the others in the practice, and takes calls if anything comes up. They also come in for an hour or two on Saturday and Sunday to write for any meds that need to be given to any patient that needs them. But its a large practice, so I think each doc only has weekend call once every few months.
At least I think thats how it worked.