On the Contact record's Office Address section, the block of fields that update with the “Use Primary Org Address” checkbox include:

• Street Address 1, 2
• City
• State
• Postal Code
• Country
• Alternative Phone
• Description

Any changes to these fields will update the Org record, as well as all Contacts with this as their primary org and have the “Use Primary Org Address” checkbox flagged. Telephone and Fax numbers are excluded from the update, but the Alternative Phone should also be excluded from the address update and treated like the other phone fields. Client would like to use the Alternative Phone field to enter a phone number not shared with other contacts.