The list of "OBiTALK Approved Service Providers" is the list of "all the providers that we specifically partnered with that use our service provider API to support remote auto-provisioning of our OBi devices".
The list of "OBiTALK Compatible Service Providers" is the list of "all the providers, for which we have some known configuration parameters, which we will pre-populate in your OBi's configuration, when you click the provider's name to select it."
Nextiva isn't included in the second list, but yes, they are in the first list. Nextiva likely wants you to use an OBi IP phone since they have invested time in integrating the phones' feature set with their own feature set. Since they target business customers, it's a cleaner, more modern, feature-rich solution to buy and deploy IP phones, vs. ATAs with analog phones.