Yes, we validate your phonebook contacts before uploading the contacts in the webapp. 

We run a check on the whole list to scrub the duplicate, invalid and NDNC/DND numbers. To smoothen the uploading process, we remove the duplicates and invalid numbers from your list. 

The list that final gets upload upload is the list after removing the duplicates and invalids. We also, validate the DND/NDNC numbers and you check/verify this in the phonebook section under CRM.