Phones - How to edit and delete contacts from Sangoma phone's Contacts application

Q: On my Sangoma phone used with Asterisk, I added contacts from the phone's Contacts application. How do I edit or delete those contacts?

A: The current Sangoma phone firmware (for Sangoma D40, D50, and D70) permits adding contacts from the phone's Contacts application, but does not permit editing and deleting contacts. There is an existing request to provide this capability.

However, see also the answer to the following, related question in the Phones - Sangoma Phones + Asterisk FAQPreview in the Asterisk Wiki:

Q: Is it possible to add contacts from the phone and have them stored on Asterisk?