Forget about messing around with apps. There’s a powerful voicemail handling system built into nearly every big carrier today — and that system almost always includes a system for recording a voice message and sending it directly without ringing the target phone. I will walk you through the process of doing this on Straight Talk (since that happens to be my carrier) but most carriers will have similar systems.
Open the Phone app. Tap the Voicemail tab in the lower right-hand corner of the screen. If Visual Voicemail is not set up on your iPhone, you'll see a button that says, Set Up Now. Tap that button. Enter, then confirm a custom voicemail password. Tap Custom if you want to create your own custom greeting when someone reaches your voicemail box.

Change your voicemail password: Go to Settings > Phone > Change Voicemail Password, then enter the new password.
Insurance agents have to deal with large numbers of clients almost all through the year. So, they are extremely busy and cannot take calls of clients. For them resorting to voicemail messages become inevitable. Here is one instance of VOIP phone service for businesses like this: