question

You could change the number type of the line by going to your RingCentral portal and under the Company Numbers & Info section, select the line and on the right side panel changed the Number Type.

Of course then you will be unable to receive ANY incoming phone calls. You don't state which type of account you have but perhaps you only need a FAX account.
http://www.ringcentral.com/fax/features/how-it-works.html

You have a few options here: 

1) There is an option for a FaxOnly Account (see your sales rep). 

2) Block calls from calls without a Caller ID. 

3) Change the number to FaxOnly