Sprout Forms: empty phone field still shows error "Phone is invalid. Example format: (506) 234-5678"
This was a bug and is resolved in release Sprout Forms v3.6.9 and Sprout Fields v3.5.2. 



Original Reply:


  This is a bug in Sprout Forms v3.6.8. We'll have a fix out for this shortly and I'll update this comment when we do.