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'.