Ive been saving my grandmothers voicemails for almost 8 years now and when i went to upgrade my phone, i almost fainted when the rep told me they were going to be erased. Thanksfully he put us in touch with a company called http://www.voicemailsforever.com and they ended up much cheaper than the one you suggested above. i think your readers would benefit because they do it for so much less.
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).

Set up voicemail on iPhone 11 is not a difficult thing, so you must have it. Through it, you will share voicemails with AirDrop. You can save them as an audio memo to save on space, mail them to others, and more. Let’s enjoy it and personalize it as you wish to get comfortable using the voicemail system.
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.