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).
From the My Phone Service page, scroll to My voicemail and select Set up voicemail. Choose your voicemail box type. Select Advanced voicemail (suggested method), and then select Next. Subaccounts voicemail access and phone number assignment. Add name and assign each phone number a name to easily identify who is using the line. Select Continue.

When someone reaches your voicemail, it’s important that you help them confirm that they have reached the right person by providing all of the relevant information that they will need. Who have they reached? Did they contact the right person and the right business? Should they leave a message? When will you get back to them? Is there a better time for them to call?
3. Connect to voicemail. This next step may be dependent on your specific phone. Some users with basic phones can start by finding the Phone icon on the main screen of their device.