How to add Phone Number Extensions in Shared Address Book?

Searched the web and can't find an answer to this question. So thought I'd try you cPanel gurus. I am using the Horde Shared Address Book and syncing it to the employee's phones via CardDav. But the system will not allow me to enter telephone extensions into the phone number fields, using established protocol for them (i.e, "555-555-5555,,,,103") but I get the following error: "You must enter a valid phone number, digits only with an optional '+' for the international dialing prefix" and it won't save. Does anyone know of a fix or workaround for this?