“Hallo”, “Hillo”, “Hollo” and “Hullo” are all very old words. They all more or less mean “Hey, over there!” or “Golly!”. I am not familiar with the “Whole be thou,” theory, but I doubt it very much.
“Hello” with an “e” was invented by Thomas Alva Edison (!) especially for answering the telephone. Bell used “Ahoy!”
Most languages use a different word for answering the telephone than for meeting someone on the street. The most interesting example is probably Italian, where “Pronto” is the way to answer the telephone, but, when used on the street, translates into something like “Hello, sailor!”. (It literally means “Ready!”)
For some reason, in English, the telephone word took over for the regular words, such as “Good morning,” “Good day,” “Well met,” etc.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams