Using the old tenants telephone number would actually be fraudulent, as you have to agree you are the account holder of that number to use it, which you would not be. So yep - they just use the address.
What would happen if the previous tenants did not notify the provider... Nothing really. It would have no bearing on the new provider for your son, who would either install a new line or take over the existing one (issuing a new landline number in the process).
If someone placed an order on your existing line, this is what would happen.
The new persons order would be accepted and they would be provided an install date.
Immediately your current provider would send you emails and letters, informing you they have been notified your service is being taken over. You will be given 10 days to contest it and say this is not valid, the current provider can stop the new persons order. The letter and emails will inform you of a day after which they cannot stop the new persons order.
The new person must also pass the ISPs credit check, that is they cannot give fake details and pass & the new account normally has to be linked to a bank account... If they were fraudulent they would be fully traceable. So the likelihood of this happening is almost zero.
So yes, anybody can place an order, but that user must pass the new ISPs credit checks, and you must ignore all communication from your current ISP for it to go through.
Edited by ukhardy07 (Sat 24-Sep-16 00:36:17)