SIP domains
A SIP domain is the name that groups the telephony objects of your account. Every extension, trunk, and outbound route belongs to one SIP domain. An extension registers its devices against the domain name. Extension numbers only need to be unique inside one domain.
Hams support creates and changes the SIP domains. The SIP Domains tab only shows the domains. You cannot add, change, or delete a domain there.

Columns
Use a domain name
If the tab shows No SIP domains provisioned yet., ask Hams support to add a domain. You cannot add a trunk, an extension, or an outbound route before a domain exists.