Here are screencaps of the dropdown

You are exiting the Sub before you Requery the form.

Is CallSign Text or a Number?

Relational Combo

Alphanumeric, stored as text.  An example would be my FCC call sign W5FJN

Assumming CallSign is in the second column of your Combo Box:

    WHERE CallSign=""" & CallSignCombo.Column(1) & """"

Double Double Quotes

It would be a good idea to switch to a proper relational Combo Box rather the Multi-Select.
You will run into more problems later.

Evil Access Stuff

Richard's Multivalued Fields video shows how to move the data from a multi-valued field into relational tables. Once you get those moved into proper relational tables, having a multi-select list box built in a form isn't bad.

Richard discusses multi-select listboxes in Access Developer 15