left. Надо было позвонить мне перед отъездом.
should’ve = past tense

Change gears
1. To shift gears in a manual / standard transmission vehicle
*very formal language
You shouldn’t change gears without depressing the clutch in a standard transmission vehicle = formal way
You shouldn’t shift gears without pressing in the clutch in a standard transmission vehicle = spoken way

Change gears
2. stop what you are doing now and switch focus to something else instead
I know that you’re working on verbs now but let’s change gears and do nouns instead for a while, OK? = stop doing something and do something else instead
I wanted to finish this document but I had to change gears and only work on the stuff that I absolutely had to get done before noon. = to change my focus and prioritize

More figurative meanings