It's actually pretty common to just throw in a couple of "Jas" every now and then. "Ja, hallo" or "Ja, guten Tag" are typical ways to start a phone conversation. Sometimes you can hear people say "ja, danke" or "ja, bis dann" instead of just "danke" or "bis dann". In this case, the "Ja" doesn't mean anything. It's not an answer to any question. It's more of a meaningless phrase that doesn't really change the meaning of what you're saying (that's why the translation "yes, hello" is confusing IMO because I don't think you can "yes" this way).
The sentence is a little bit strange. I can imagine two situations in which a sentence like this can be used:
a) at the phone: If someone asks you if xy is speaking and you answer Ja. Hallo.... But ins this case I think it had to be two sentences: Ja. Hallo. (and also a little bit unpolite..)
b) If you see someone you haven't seen for a long time you can express your surprise talking to him with, Ja, hallo!