The Exchange Line Site Assurance (ELSA) service comes as part of BT's ISDN30 service, and enables you to divert inbound calls to your organisation according to up to five dial plans. You need your ELSA plan reference and ELSA password to invoke disaster recovery services in the event of a disruption.
The ELSA plan reference is provided by your service provider, for example, BT. Contact your account manager for details of your ELSA plan reference.
For more information, refer to your smartnumbers Voice Continuity service guide: