I've found a way to enable a new PC to call and receive calls using RC for Windows (softphone)from any available extension, not just the main extension 101.
When logged in to your company account on the RC webpage (ext 101), go to the "phones" tab, "disable" the existing PC if it is not the one you want to have "activated" with a fully functional softphone.
Then, using the PC you intend for the full function Softphone, log into Ring Central for Window app (Softphone) using the main extension (x101). This will make this PC show in your account as authorized to make and receive calls using your digital subscription.
Then log out of the Softphone and re-log in using whatever extension you desire.
Now that Softphone will be able to make calls (using a headset) or receive calls at that intended extension instead of only accepting calls for extension 101.
If you have a desk phone, it will continue to make a receive calls reflecting your main number/extension, not the extension that you just enabled for the target softphone above.