Our domain/realm is sip.sip-trunking-services.com but you have the option instead of using your own domain DNS C NAME record that points to sip.sip-trunking-services.com.
If you do use your DNS record, do contact your domain provider and ensure the TTL (time to live) is set to be short (2-5 minutes). Importantly, please ensure that your local devices (firewalls, gateways) do not cache DNS settings. Ensuring this is done will mean you benefit from our auto-failover service and ensure that any failover happens within a few short minutes. Please also refer to Registration Timeout Settings and Recommended DNS / Name Servers.
Please note that the domain/realm you use to register with must be the same as the 'SIP Domain' set in your reseller 'Options' page. You can change this setting but in doing so please that you should ensure your trunk settings reflect the change and importantly that you reset trunk passwords.