You may need to call an ANI- Automatic Number Identifier. You can also try calling the following numbers: 1010732-1-770-998-9664 (AT&T ATL VPN Test Number), 1-866-692-6447 (Q West), 1-800-444-4444 (MCI); these will direct the call to the port where you can find out your number.
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To find out your own landline number, call the customer service number for your telephone service. Select the 'speak with a customer service representative' option. You could also check your monthly billing statement or call another land line which subscribes to a caller ID service.