Users have issues to start voice or video calls since the visible UI when their partner changed their call's privacy settings is too technical for the average use to explain and to be understood by the other part.
Tapping a call request button in the error popup would ask to the user to send to their partner a predefined help message:
'I'd like to be able to call you on Telegram. Please review your [calls privacy settings].'
A internal tg://settings/privacy/calls link or a link to a tips channel post (https://t.me/TelegramTips/89) could be included.