To add custom field to a contact list you got to Contacts Manager > Lists

Then you click the three dots on the right of the list you would like to edit and choose edit list

Then go to Custom Fields

To add a new one press the plus sign in the right upper corner

Settings of custom field:

Alias: Name the custom field.

Type: What type of information is in this field? (Do not choose Number for phone number as this will remove leading 0 so 0771121210 will be 771121210)

Required: Make the custom filed required or optional when creating a contact.

Click to dial: Make a call button appear next to the field for agents (Only works if Type = Text)

Read this article on how to create Global Custom Fields.