Clients often ask me how to track bad phone numbers in REPS. You don't want to just delete the phone number because you want a record of the fact that it is no longer in service. Otherwise it would be too easy to accidently find it in your notes or online one day in the future and think it's the right number.
The best way to track this information is NOT with an activity record. You don't want to use "Disconnected Number" (or something similar) as your result code of the last attempted call because the next time you think about calling the lead, you might not look at each historical activity before you start dialing. And what if you printed out a phone listing and didn't have access to the activities as you were dialing? You need a more fool-proof system.
The best way to keep track of a bad phone number is to add a phone number type called "Not in Service" and choose it for the number that no longer works.
Second, move the "bad" phone number down to the Phone 4 field, so it doesn't show up when you select "Contact Phone" in the Field Selection list or when you run a Name/Address Listing report.
By moving it from the Phone 1 field and tagging it as "Not In Service," you'll instantly be reminded not to use it when you look at the record and it won't accidently show up when you're exporting phone numbers. Problem solved.