In Australia it is much more common to say "Where is my phone?" than any other translation. If you are looking for your phone, the portability is assumed.
I think "where's my phone" is a more natural-sounding translation in pretty much any modern English-speaking country, particularly given that the fact you're looking for it implies that it's not a landline. "Cell phone" is definitely not British English.
As a speaker of British English, it seems hard enough to learn French without having to think what the translation would be in US English.
And this is not the only exercise where this applies ...
In British English a "téléphone portable" is a mobile phone, a mobile, or just a phone. It is not a cell phone.
Portable phone is much more common in English speaking Europe. Cellphone is an american mis-interpretation again.
Isn't a term "smartphone" already synonym for cell phone? I can't imagine anyone who has different style of portable phones but smartphones.
I don't have one of those smarty phones, do not like how they fit in my pocket. A flip phone is good enough! I can have it on me instead of leaving it in my car or in my suitcase.