Context does matter here. If the caller would reasonably expect you to answer the phone (such as if they were calling a store or other place of business), an apology for not being able to get to the phone makes the most sense.
Click Save to save your external number. Repeat this step for any additional mobile numbers. Once all agents have been added, click Save on the Ring Group.

Obviously, I need to update it. And if you haven't changed your voicemail greeting in over a year, you're likely in the same boat.
The phone you use to record your greeting – and your surrounding – can turn your carefully scripted greeting into an unprofessional mess. Background noise is terribly distracting, so choose a quiet room or parked car to make your call. Landlines, or a “wi-fi enabled” cellphone call, can provide much better connection quality than a standard cellphone. If you must use a cell phone, be sure to use a high-quality headset for the best clarity.