You might also consider giving them the option to skip your voicemail message completely by pressing another key (again, check with IT). You also want to make callers aware what you will return their call, instead of telling them that you will try to return their call--and let them know when you are likely to do it, if possible. If necessary, provide additional or emergency information in your voicemail greetings. My dentist, for example, lists his cell phone number for emergencies.
My iPhone rings for 15 seconds – which is actually eight rings – before it goes to voicemail, but even that’s not long enough if I’ve put the phone down in another room or it’s zipped into a pocket in my jacket.

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With notifications, you're alerted when you miss a call or receive a voice mail message. You can decide how and when you want to be alerted. For example, you can receive an email when you miss a call or get a text message when someone leaves a voice mail. By default, you'll receive an email anytime you miss a call, but text messaging notifications are turned off.