Hi Traveler-kk,
welcome to this Apple (user-to-user) Community!
Re: "Assistive Access, need voicemail access!"
Wonderful to know that your father-in-law has your help!
You may want to check out whether or not access to VoiceMail is possible from within Assistive Access.
As far as I know it hasn't been possible in the past. However, I don't know if anything has changed recently.
Apple's accessibility experts can likely tell you for sure.
How to contact Apple for help with accessibility features - Apple Support
Seems Assistive Access keeps everything as simple as possible, yet you can add some apps in original form.
If you haven't yet seen it: Add, remove, and customize apps for Assistive Access on iPhone - Apple Support
and the Assistive Access User Guide - Apple Support for iOS26 may be helpful.
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If needed: You can also send a feature request to reach those who work on Apple's accessibility software:
To provide feedback about an accessibility feature on an Apple product,
you can email the Accessibility team: accessibility@apple.com.
All the best :-)