I wish that were the solution, but if your cell phone is turned off, out of range, or in airplane mode, the US Mobile voicemail kicks in immediately, without even trying to ring the phone. Google Voice doesn’t have a chance.
Forget about messing around with apps. There’s a powerful voicemail handling system built into nearly every big carrier today — and that system almost always includes a system for recording a voice message and sending it directly without ringing the target phone. I will walk you through the process of doing this on Straight Talk (since that happens to be my carrier) but most carriers will have similar systems.

For small businesses, there are a variety of voicemail service providers you can use that offer different features. From transcription and voice to text to more convenient features of handling multiple phones through one number, availability varies by service. Shared voicemail and call routing is intended for multiple access and directing calls to one or multiple phones. Mobile apps are also available that allow you to check your messages from your phone. The below infographic outlines the current state and trends of voicemails.
If you are recording a voicemail message for business, be sure you include your name and your company's name, so people know who they are calling and were they are reaching them. If you do not include the business name, they may think they have mistakenly called you at your personal number. You will also want to make sure to give callers the option to speak to someone who is currently available, an operator or other representative, by pressing a certain key (check with IT personnel to see what this might be).