Jan 21, 2008 · Where "(your phone number)" is your 11-digit phone number (including 1 and the area code), and "15" is the number of seconds (from 5 to 30 in 5 second increments) that you want your phone to ring before going to voicemail. (The default seems to be 10 seconds.) You can thank me later. 😉 (Hat tip to Bradley!)
If you have voicemails saved on a non-smart phone, your best bet is to record them on a computer. This is a method that requires some equipment, but not as much as you might think.

If your carrier supports the iPhone voicemail feature, you can retrieve and listen to your voicemail right from within the “Phone” app and call your contacts back by simply tapping their messages. That type of functionality is pretty neat and useful, because it saves you actual time. If not supported by your carrier, you will not be able to see the visual overview of your voicemail inbox and use any of the related features. Tapping the “Voicemail” tab in your “Phone” app will simply call the number associated with your voicemail box. So let’s get going with creating our custom voicemail greeting on iPhone! How to record your own Voicemail on iPhone Phone → Voicemail → Greeting → Custom → Record → Save
There are two tiers of paid accounts, giving you ad-free access, the ability to “Slydial” multiple people simultaneously, and other premium features. For most people, the free version is perfectly adequate.