You would think there'd be app for that, but Apple doesn't allow third-party apps to access the microphone and the integrated Phone app directly. However, there are tricks within some apps and services to get around that limitation that can make you a mobile wire-tapping master.
The telephone systems have the feature named as Voicemail. VoIP is the best example of it. When a receiver is unable to pick up the phone, then the caller can leave the voice message on the receiver’s phone. In older telephonic machines, such messages were stored as a voice message directly, and the receiver can hear them later. In modern telephone systems, such info is being stored on the server of the service provider in the user’s mailbox. It is similar to email in which the messages are saved in the form of audio instead of text.

To get the most out of your voice mail it’s important that you properly set up your personal mailbox. Press the voice mail button on your phone and enter your password (0 0 0 0 by default). Press 5 from the mailbox menu. Press 1 to assign your no answer greeting Press 2 to edit the greeting, begin recording when prompted. A typical greeting says “Hi, you’ve reached Mitch Scott. I am currently on the other line or away from my desk. Please leave your name number and a brief message and I will return your call as soon as possible.” press # when finished. Press 1 to set this as your active greeting Press 5 to edit mailbox greetings Press 7 to record your mailbox greeting and follow the prompts to record your mailbox greeting (You can repeat the greeting you used in step 3). Press # when finished. Press # from the main menu Press 7 for personal administration Press 2 to record your name “Mitch Scott” press # when finished Press # from the main menu Press 7 for personal administration Press 3 to enter your directory name Press 2 and follow the instructions to enter your name in the directory Press # from the main menu press 7 for personal administration Press 1 to change your password Press 2 and follow the instructions to change your password.
5. Use voicemail on a computer. Download messages to a computer. If you're the Primary Account Holder, you can save your messages from your My-T-Mobile account.