Whoops error when saving UCP button config for Sangoma Phone

Steps to reproduce:

• Have Sangoma EPM template with "Voicemail Key Type" set to "Standard Voicemail"
• Browse to Extension Mapping and click "See UCP Custom Entries"
• Select an option that includes "Rebuild config" and click Apply

Action fails with this whoops error:

Whoops\Exception\ErrorException
Undefined index: dialvoicemail
File:/var/www/html/admin/modules/endpoint/EndpointSangoma.class.php:2946

Work around is to save the settings when editing the button layout and then rebuild from Extension Mapping, or set the Voicemail Key Type in the template to Voicemail Application.