While they are listening to your voice, they are internally deciding whether or not it is worth their time to continue or hang up. Look at your voicemail message as its own short advertisement.
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Voicemail service providers like Voicemail Office offer these life-simplifying features as part of their unified messaging packages, which allow users to obtain both voicemail and fax messages in their email inbox. Voicemail messages that are sent to email are also identified with the extension number in the body of the email, so that the purpose of the call can be foreknown. Surprisingly, rates for these services are very low, starting at just $9.95 per month.
The problem there is that VoIP traffic is much more sensitive to network bumps and potholes than most general office traffic. That translates to garbled conversations, difficulty connecting over Wi-Fi, or (worst case) dropped and lost calls. If your business is small and your network is essentially contained in one or two wireless routers, then your configuration and testing headaches might be fairly easy (though still there). For medium and larger networks, these tasks can be complex and time-consuming, which translates into added cost in terms of man-hours.