Contextual validation error message missing

After investigation the problem lies in a change in the way CF7 targets fields targeting starting with version 5.6.

Before: the field name is added as a class
After: the field name is added a data attribute

Plugins that add form tags to CF7 must change the way they build the HTML markup in order for the validation to work. With International Telephone Input for CF7 this means modifying line 90 in includes/form-tag.php

Before:
'<span class="wpcf7-form-control-wrap %1$s"><input %2$s /><input %3$s /><input %5$s /><input %6$s /><input %7$s />%4$s</span>',

After:
'<span class="wpcf7-form-control-wrap" data-name="%1$s"><input %2$s /><input %3$s /><input %5$s /><input %6$s /><input %7$s />%4$s</span>',

Obviously changing the code in the plugin yourself is not a very good idea. Ideally the author should validate this kind of changes, make sure it doesn’t break anything else then publish a new release.