I have seen people do both roles. Depending on how much you end up getting up at night, going out at night, how far you have to drive, the weather and most of all if you get "stuck" with admissions, recerts etc. I have seen agencies give the "on call person" a bunch of working visits. So for example, if your on call Friday-Monday AM, and there are 5 admissions and no one else to do that, the on call person gets stuck doing that AND taking calls/call visits. I have also seen huge coverage areas and in the dead of winter, that stinks.
I would also find out if you are responsible for staffing or calling staff to cover call offs. If an agency gives the on call nurse, the duty of staffing call offs, it can take alot of time. Also I would find out that if you cannot get someone to cover, ask i f you are required to do all the unstaffed,call off visits.
I hope this helps,