A digital recorder is nice and all, but if you plug a recorder directly into an iPhone using a 3.5mm audio cable, you're not going to hear the call. Using the iPhone headphone jack—assuming your iPhone is so old that it even has one—cuts off the speaker. Get the Recap-C, a $99 adapter that plugs into an older iPhone's 3.5mm jack, with output to a headset as well as to a recorder. The secondary recorder—connected via a 3.5mm male-to-male auxiliary audio cable—is up to you. It could even be another iOS device (or Android or PC, but stick with the digital recorder for simplicity).
Businesses should have two main types of greetings to create an excellent call experience. The first one is what we call a “welcome greeting” – this greeting welcomes callers to your business. Typically, a welcome greeting will present a menu of call options like hours, location, or customer service. For more details on how to create a welcome greeting for your general business number, read this article.

Most of the time, an update to your carrier’s voicemail app or settings will cure the issue; however, it is important to confirm that your voicemail number is appropriately configured before proceeding any further.
Of the many things your phone can do, automatically recording a phone call is not one of them. Perhaps not including that feature is a security precaution for phone makers; it could be a potential liability issue. Regardless, from time to time, documenting a call is a useful thing to do, whether you’re conducting a phone interview or cataloging a customer service call.