Is there any difference of meaning/usualness between the two sentences below?
- Tu rentrais quand tu m'as appelé.
- Tu rentrais chez toi quand tu m'as appelé.
From some OP's comment, I understand the question being more about the use of rentrer in French compared to the use of come back in English.
When tu rentres as well as when you come back, you physically always rentres quelque part and come back somewhere
quelque part and somewhere are not necessarily explicited. The location will then be inferred from the context.
If the context does'nt help then, one will resort on defaults. And here is the difference between to come back and rentrer : Their default implicit locations differ.
When you use to come back (without any precise location) the implicit location defaults to : Where you are now! So, if you mean anywhere else, you are forced to be explicit. Therefore, if you go home, you are somewhere else and can't just come back but must come back home.
In that precise meaning of come back (without home), french would prefer using revenir or retourner to rentrer.
With rentrer, The default location is : the place where you usually live / have your habits. (TLF A.2.)
Which implies that when the context mentions any entrance of anything else than your home, or if there is a fair level of uncertainty regarding the place you live in (chez toi / moi / tes parents / à l'hôtel...) you should prevent potential ambiguities and be explicit about the location : rentrer chez toi. If there is no potential ambiguity, then, you can resort to default and only say rentrer