"– Hello? Where are you? – In the metro."
Translation:– Haloo? Missä sinä olet? – Metrossa.
I don't think it would sound weird, no. Especially with mobile phones, where it can be generally assumed you know who is calling. Answering a friend or family member with e.g. "moi, mitä kuuluu?" ("Hey, how are you?") would be perfectly normal I think.
At least I partially associate "Haloo" with answering the phone when you don't know who the caller is. There are other options to answer in those situations though, such as just saying your name, or your name + "puhelimessa" (literally "in the phone" if we consider -ssa as "inside", but means more like "speaking").