I read that it is used in telephone calls. Is there anyone who uses it like that, or has heard it be used?
I'm wondering whether it would be suitable when coming into a house, calling to see if there's any body there.
There’s a cool translation of a song by Adele called “Ha Lo”. I believe the song is supposed to be about the singer trying to contact an ex-boyfriend by a long-distance phone call. She repeatedly says ha lo to start the conversation:
Do ha lo el la kontraŭa flank’
Eble por la mila foj'
Por al vi pardonpeti pro ĉiu faritaĵ’
Sed vi ne estas hejme ĉe ajna mia vok’
The Reta Vortaro confirms this definition too:
ha lo Duvorta ekkrio, kiu estas uzata kiel alvoko, precipe en telefonado.
I think your suggestion of using it to check if anyone is home is suitable too.
Ha lo is used this way in Mazi en Gondolando when Karlo is getting all the phone calls asking if he has time to do something. I believe Reĝo also says it when answering the phone.
It would seem to me a totally normal thing to say when calling into a room to see if anybody is there. The entry in PIV for lo seems to support this as well, saying it's an interjection used to get someone's attention.
There are 5 hits for Ha lo in the tekstaro and 4 of them seem to be related to telephone conversations.
On the phone, you usually use ha lo, to call as well, but this is less often used. Ha lo does not mean hello used as a welcome; in that case, saluton is used.
En la telefono oni uzas kutime "ha lo", por alvoki ankaŭ, sed tio estas pli malofte uzata. Ha lo ne signifas la anglan "hello" uzata kiel bonvenon; en tiu kazo oni diras "saluton".