Large lists of contacts can be deleted in a couple of ways:

Imported File

Import a CSV file containing the mobile numbers to be deleted. After the file has completed processing (est 10-20 minutes depending on it's size), click the delete icon for the desired file in the Import History menu. This action will delete all of the contacts on the file. 

Note: If the list being uploaded for deletion is too large it may time out before the import can be completed. If this occurs, please open a ticket or send an email to support@ with the file of contacts to be deleted and the client ID where the deletion needs to take place. 

Contacts Table

The contacts table of every account allows for deleting contacts listed as PENDING CONSENT, PENDING OPT-IN CONFIRMATION, OPTED-OUT CONTACTS, and NON-SUPPORTED CONTACTS, as well as ALL CONTACTS for the account. Click the delete icon for the desired group of contacts to be deleted.