a call to my sister

Yesterday I made a phone call to my sister.
That was my first time to call her since she was abroad.
Someone answer:” Hello?”
The tone sound like a foreigner.
It’s her cellphone, I supposed she might answer so I use Chinese.
“Hello?” I was nervous. Oh my god, what should I say?
“Is Jo there?”(Jo is my sister’s name)
“Oh, I’m Jo” After few second…”Are you my sister???”
Haha, I can almost see her surprise with pleasure!
She was in Vienna on vocation because her school hope they could get acquainted with their capital.