How should I decide between "sometime back" and "sometimes back"?
Sometime back I received a call from Mr. X
Sometimes back I received a call from Mr. X
Which sentence is correct?
'Sometime' refers to a single instance, which fits with the single telephone call.
'Sometimes' refers to a number of instances, in which case you would need to refer to more than one call, but you wouldn't use 'sometimes back'.
You have a typo and a missing article. The 1st phrase should be 'Sometime back I received a call from Mr X'.
You can also use 'A while ago' - 'One day/night'.
For your example the best choice would be
Sometime can be correctly used to express some important (for the 1st self) things that happened or will happen.
It can also be used as an adjective meaning happening previously, but not now:
And finally, always remember that there are three different versions: