My sales team is saying that call histories are missing in Salesforce. Could this be a result of uplaoding lists? I know that Marketo overwrites Salesforce; however, to prevent this, I only upload lists with first name, last name & email address. Could the uploads be affecting other portions of data? If so, how can this be prevented?
No, Marketo should not be deleting tasks, and certainly would not do this via a list import. Marketo can only create Tasks.
Where are you keeping Call Histories? Salespeople should do this ONLY in Tasks. I bet they are using the Description field, which is the Person Notes field in Marketo. This can be overwritten by Marketo or other methods.