It turns out there is not a built-in way to accomplish this in the RingCentral for Desktop app. There may be a way to implement this though. If the administrator of your Filemaker database allows Filemaker to format phone numbers properly, and if you view the database in the Google Chrome browser with the RingCentral for Google plugin, you might get it to work.
Every implementation of Filemaker is different, so it's hard to say if this would be possible with your database. I'm not knowledgeable in Filemaker to say myself, but have your Filemaker admin check out the links I found above and see if this could work for what you have.