You can say:
Zdravstvuyte, mogu ya pogovorit' s Kristjanom, pojaluysta?
Здравствуйте, могу я поговорить с Кристьяном, пожалуйста?

Here's an audio record so you can hear how to pronounce it approx. They will understand you no doubt. And this is the most polite and well-mannered asking.