„eine“ is not an indefinite article in this sentence. Its the number „one“ in this example:
In ONE hand she was holding the phone, in the OTHER [hand] she was holding a key.
Its the phrase
Die eine.... die andere...
Another example would be:
Die eine Seite der Münze zeigt eine Zahl, die andere [Seite] ein Wappen
Yes, it's perfectly fine.
You shouldn't consider einen as an indefinite article in this case. In English you would say "In the one hand" - and not "In the a hand". The einen has a different grammatical function.