In FAQ about Phone Post is written: Choose a Personal Identification Number -- this is not the same PIN that is used for posting via email. You must also specify your phone number. This is used to authorize you; we will never use this number for marketing purposes. If there are multiple people in your household who share a phone number and intend to use this feature, make sure that you choose different PINs.
And then: When you call the Phone Post number, you will be asked to log in -- either by caller ID if you are calling from your designated phone post number, or by entering your phone post number if you are calling from a different phone number. What is caller ID - do you mean my phone number displayed on your phone or PIN or smth else ?