Agree with the excellent advice so far. I would add that if you are calling a senior resident, do the following:
1. Always see the patient, do your own physical examination, and for God's sake, take a fresh set of vitals. Nothing is more irritating to hear on the phone than, "Well it looks like the blood pressure 6 hours ago was..." You are the guy/girl on the seen, and there is no excuse for not doing your own examination, and taking a fresh set of vitals.
2. Before you call, have a plan, and a diagnosis. You might be wrong, but at least think about what you are looking at, make a presumptive diagnosis, and formulate a treatment plan. The sooner you start to do this, the better you will get at being a doctor. Do not call unless you have thought these doctorly things first, so that you can have an informed discussion with your senior. The conversation on the phone will go a lot better, you will learn a lot more from the exchange, and you will solidify your reputation as a good resident.
3. Cannot agree more with the advice to document every patient encounter with a time-stamped note in the chart. Even if you miss something, you will be forgiven if you saw the patient and documented your findings. If the patient codes 30 minutes after you saw her and did not document your normal exam during your visit, you will be in a tough spot.