Many people are confused by the term “default provider”. This is basically set aside for 911 use, more than anything else. It does not prevent you from calling other VRS providers, such as ZVRS.
You need to register with a default provider who usually is the company that provides you with a videophone, so that your location information can be saved with them for 911 purposes. In case of an emergency, you can dial 911 on your videophone and it will connect you with the default provider who will have your location information already given to them previously.
If you want to make a VRS call (that is not a 911 call) or receive a VRS call, you do not necessarily have to use your default provider for this purpose. You can connect directly with the VRS provider (in the case of ZVRS, you can call 888-888-1116, and you will be connected with ZVRS.) If you have a Z340 for example, and the hearing person calls you using your Z340 phone number, it will connect them to ZVRS agent who in turn will call you on your Z340. This has nothing to do with being a default provider.
Remember—default provider=911.