Make phone type value "mobile" for every individual
You might be running the manual query under a different mysql user than the one that owns the function, so you can check who the "definer" is using show create function civicrm_strip_non_numeric;.

If it really is "missing", then you can also manually recreate the function - it's defined in CRM_Contact_BAO_Contact:
https://raw.githubusercontent.com/civicrm/civicrm-core/7f8858ced29deeabc720e1c37c42e1b7d59d8962/CRM/Contact/BAO/Contact.php

[While I first did not succeed in recreating the function, the suggestions below led directly to the desired outcome. -- Prónay]