I don’t know if the lookup works with custom attributes but I know it doesn’t work with phone numbers that are shorter than 4 digits. So if an internal caller is only signalled with 3 digits then it will not work. I solved this by transforming caller ids from internal callers to the international standard format in our zammad cti script on the pbx server. I also just renamed the Fax attribute to Phone 2 as there is no need for a Fax number and stored the internal numbers of our agents in international format there for the lookup.