I imagine that situation like when you're talking on the phone. When you first answer the call.
I imagine a job interview, they call out your name, yeah, hi, nice to meet you or whatever haha
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).
Bizarrely neither hallo nor hullo are accepted, but both are acceptable variants of hello in English. I've reported it.
Maybe this is a difference between American and British English. I can't think of a time Americans ever normally use those variants of the word hello, except when they're pretending to be British.
You're right, I should have said 'in British English". But duo should still accept it, otherwise us Brits get dinged a heart when using our mother tongue, which seems a bit unfair!
So is this basically equivalent in meaning to English's "Yes, hello?". That is, the "Yes" is a prompt for the other person to explain what they want. For example, someone walks up to your desk and you say "Yes?"
Or is "Ja, hallo" really interchangeable with "Hallo." ?
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!