The default voicemail greeting on the iPhone plays generic Your call has been forward to an automated voice message system recording. If you use your phone for work, create a personalized greeting that tells people you're a professional. If your phone is for personal use, create a personalized greeting so people hear your voice and know they called the right number.
When your phone only rings once then cuts out, this fault is known as "ring trip". It can be caused by a number of things either inside or outside your property, and will require a process of elimination to track down the cause of the fault.

A professional voicemail greeting is essential in many aspects of a business. In sales, it enables you to properly respond to inquiries from prospects and customers or discuss a deal in more detail. When they call, and you can’t answer, you may lose the opportunity for immediate communication and potentially lose a client or customer.
After you hang up, the recorded call will appear in the Voicemail section of your iPhone’s Phone app. The Easiest, Most Versatile Method: Just Use Speakerphone and a Recorder. At the end of the day, the best and most simple method for recording calls on the iPhone is doing it the old-fashioned way: Speakerphone and a recorder of some kind.