Apple ID Phone # verifications

Our company has many iPads that we give to our field guys. We issue each person an Apple ID that is associated with their work email. We don't want to connect their personal phone numbers to this account in case the iPad needs to be reset for a new user and they no longer work for us. We were using our office line for the phone number to receive verifications but now that isn't allowed since that number has been used too many times. Any suggestions for what to do?