There is no "in general" answer to the question. Each program is a bit different.
We have 2 research years. One year is completely protected from clinical responsibility and call (6th year). The other year (3rd year) you are not in the general call pool but you do cover call when one of the clinical residents is on vacation and have one schedule call per month.
Our program is like this with 2 residents per year:
PGY-1: 6 months of neurosurgery with q4 call. Other 6 months on other services.
PGY-2: junior resident on neurosurgery; q4 call
PGY-3: research / pathology / radiology; one call per month plus vacation coverage
PGY-4: 6 months peds neurosurgery, 6 months neurosurgery at the VA; q7 call
PGY-5: 12 months adult neurosurgery, q14 call
PGY-6: full research year; no call or clinical responsibilities
PGY-7: chief year; no in-house call