I think there is a bug here.
When I go to Accounts → Customers → Sip Devices and click “Create SIP Device” the account dropdown list only lists customer accounts. Providers are not listed and I can’t create a SIP device.
However, when I create a new provider, it gives the option to create a SIP Device in the initial setup but if I want to edit the created provider sip device later, the dropdown list again does not offer to save the sip device to the provider account.