How do I find my iPhone with my iPhone offline or not working?

You physically look for it. That is the only way.

You could call the taxi service to see if a phone was found.

Dm26_7

"How do I find my iPhone with my iPhone offline or not working?: [...]I lost it in a taxi and I didn’t check the registration number."

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View the Item's Info:

There is no registration number. So, view the Serial Number and such.

Go Here: Check your Apple ID Device List to See where you're Signed In - Apple Support

1. Select: the iPhone
2. View: the Serial Number

Note: Consider using a computer to perform this, as it would be more user-friendly doing so.