'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.
Sometimes, instead, means " occasionally, from time to time, now and then, etc. " If you try to replace it now, it won't make sense: I should finish this work sometimes/from time to time next week. This last one should be used for example to say: Sometimes I like taking a walk. (which becomes) From time to time, I like taking a walk.