Subform to Subform Filtering (Access 2003)

[syraqueues]

I have a database set up to track Prospective Customers, the Contacts (Individuals) within those Prospects, and Calls (Telephone, email, meetings, etc.) that I have had with those Contacts.

I have a Prospects form that contains two subforms - one for the Contacts and the other for the Calls.

The Prospects and Contacts forms are working well and the Calls subform works well when displaying all of the calls made against a Prospect, regardless of the Contact.

What I would like to do is display a button on the Contacts subform that would apply a filter to the Calls subform so that the Calls subform would display calls made only to that specific Contact, within the Company.

I found my way here via this thread:
https://www.pcreview.co.uk/forums/thread-2539615.php

And it almost answers my question except that it concerns filtering the data from the main form, not another subform within the main form.

Thanks in advance for any suggestions!