You can use the Dataform module for that. In this module you can design the activity and the submission form as desired.
For instance, for 6 calls, you can set the activity to require the student to submit exactly 6 entries, one entry per call. Each entry form can have a text area or editor field for the student report. You can even structure the report by breaking it down with further fields for various input types. If you want to allow supervisors to add their feedback to each report, you can add to the entry form fields that the student can see but not update and only the supervisor can update. Finally, you can set the activity completion by number of entries, so that it is shown completed once the student has submitted the 6 entries.
If you wish to give it a try and have further questions about the module, we continue the discussion in the Dataform forum (https://moodle.org/mod/forum/view.php?id=8192) where you can find other ideas about what it the module can do for you.
hth