Rosetta Stone taught me "guthán" for phone instead of "fón", is one preferred over the other in practice?
It's dialectal. I believe guthán is more in Donegal.
I would prefer 'guthán' because it's not a loan word. It's cognate with 'guth' meaning 'voice'.
Isn't this a bit of a run-on in English?
Your question prompted me to try and split the sentence up so it might not be a run-on, but I couldn't think of a way. Ended up sounded like a 4th grader: I am jealous. I don't have a cell phone. She has a cell phone."
I think even some punctuation in the original sentence would do the trick.
I am jealous because she has a cell phone and I don't.