After Googling this one “stukkend” (my SA buddies will know what I mean), I finally resorted to making a number of calls to get down to the actual atatchment size – cross referenced that with the math and came up with the answers.

Firstly it is worthy of note that the actualy size will depend on the following three factors:

  1. the message duration
  2. the audio codec used
  3. the audio storage format

UM uses either WMA, GSM 6.10 and PCM for creating the voicemail files.
WMA is always delivered as a .wma file and the GSM and PCM is delivered as .wav

Comparing the formats.

Format

Bytes per 10 seconds

30 Second

60 seconds

Description

PCM

160,000

156 KB

937 KB

Highest Quality, largest file size

GSM

16,000

48 KB

96 KB

Widely supported

WMA

(Default)

11,000

39 KB

65 KB

Most compressed. The WMA format has a much larger header section than WAV (about 7K compared to less than 100 bytes) WMA files become smaller than GSM from 15 seconds and up.


The bottom line…
I generally use GSM since not all devices and apps support WMA. Usually I allow for approximatly 100 KB per minute of voicemail.
Now for the tricky bit, how much voicemail to cater for?
According to Forbes (and chatting to many folks over the years…) the average person gets around 2 voicemails per day. Most voicemails are around the 30 second mark in duration (or less)
Calculation is thus approximatly 1 minute of voicemail per day per user.

Using GSM, thats about 100 KB per day per user, 3 MB per month for each user and thus 36 MB per year.

Is that all? On average yes…there are exceptions, very LARGE ones as this is based on users who actually attempt to answer calls.

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