If voice mail options are grayed out, you're using the Skype for Business voice mail service with cloud PSTN. Voice mail options aren't available in Outlook.
You don’t want to answer calls 24/7 (unless you’re serving clients globally and there’s an expectation of 24/7 support). If you’re unavailable during specific hours of the day, use the voicemail examples below for after-hours business calls.

I had the same issue and was able to figure it out. When in phone mode, press and hold the 1. This will dial into your voicemail system the "old school" way. Here you need to follow the prompts to set up your greeting for the first time. You can change other settings here as well. Once you have created an outgoing voicemail message, go to the voicemail menu on the iphone, choose custom and press play. You should here the greeting. Press save to verify it. Jun 30, 2007 6:09 PM View answer in context first Page 1 of 1 Page 1/1 last User profile for user: Cameron Jewell Cameron Jewell User level: Level 2 (460 points) Jun 30, 2007 4:52 PM in response to Dave Low In response to Dave Low
Keep it Brief. Time is money in business, so if your voicemail is long, chances are the caller will simply disengage before the cue to leave a message even sounds. Keep your voicemail short. The most should be around 30 seconds, and even that’s pressing it. We’ve grown into a country where our attention span can last only about 10 seconds in some cases, so keep it short, brief and to the point. Persuade and Engage the Caller. If someone calls your business, they already have an intention. It’s the quality of your voicemail that plays a factor if they consider you a company they’d like to deal with. This is your chance to motivate someone to engage in a meaningful conversation with you. If your voicemail is shoddy, chances are they’ll hang up, or lose motivation to continue the call. In other words, your voice mail must engage and entice someone to the point that they care enough to leave you a message.