First of all, a single extension can have multiple digital lines. As you login you account admin portal, you can see that you can add more phones (digital line with a phone number) to an extension.

In your case, I think you can create your own SIP phones then select them when you add more phones to the extension you want to have "multiple lines". However, you will need to purchase new phone numbers for the SIP phones you just added.

To first verify if you can create multiple SIP devices for a single extension, you can run a quick test by using the same user extension credentials to register for multiple SIP devices.

I think I am looking for the opposite of what you are suggesting.

I want one digital line or inbound phone number to handle a large number of calls. All calls come in on the same number. Then one SIP device will handle all the calls. The thing I want is to be able to use one SIP device and have lots of calls so I don't have to manage multiple SIP devices and transfers calls to other devices when it runs out of lines.

Having more than one phone number or extension send calls to this one SIP device could be beneficial but I only want one SIP device managing the calls that come in.