Please try the following instructions:
If running Talkatone on Android:
On Talkatone, go to your contacts, tap on the contact to set as Favorite, tap on the Menu button (on the device), tap on Open Contact option and tap on the Star icon next to your contact. The contact will appear under the Favorites tab.
Other way to do this is by marking the contact as favorite directly on your local Phone Book. Restart the app, and the contact will show as favorite on Talkatone.
If running Talkatone on iOS:
On Talkatone, go to your Contacts page, tap on the Phone icon next to the contact’s name. This will open your Contacts info page; and tap on the Star icon. The contact will appear under the Favorites tab.