One of the more straightforward requirements in the Organization Validation process is Telephone Verification. Do you have a telephone number listed? Is it verifiable by a third-party directory? Boom. Done.
What is Telephone Verification?
In order to satisfy the Telephone Verification requirement, you must have an active telephone listing that's verifiable by an acceptable online telephone directory. The listing must display the exact same business name and physical address as you've already had verified.
To check this, the first place the CA's will go are the Online Government Databases. If your number is listed and the organization name and address match up, you're good to go—you've satisfied this requirement.
Unfortunately, the majority of government databases don't display the phone number. This complicates things a bit for the CA, but not for you, as long as your organization does have a number listed.
Alternative Methods
If the Online Government Database doesn't have your organization's phone number listed, you still have other options:
Any of these methods will satisfy the Telephone Verification requirement.
Keep in mind though, your telephone listing must be up to date, and your organization must – even if just temporarily – display the listing publically.