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18 It’s not actually the carrier that’s offering this; rather it’s a capability of the PSAP. E911 is also used on PSTN |
trunks, but since that happens without any involvement on your part (the PSTN carriers handle the paper‐ |
work for you), you are generally not aware that you have E911 on your local lines. |
126 |
| |
Chapter 7: Outside Connectivity |
Conclusion |
It is generally predicted that the PSTN will eventually disappear entirely. Before that |
happens, however, a distributed mechanism that is widely used and trusted will be |
needed to allow organizations and individuals to publish addressing information so |
they can be found. Any voice technology that does not use the PSTN is currently |
either a walled-garden proprietary product, or is the playground of spammers and |
criminals. We suspect the PSTN may be around for a while yet, and if so, then origi‐ |
nation and termination will need to be part of your Asterisk system. |
Conclusion |
| |
127 |
CHAPTER 8 |
Voicemail |
Just leave a message, maybe I’ll call. |
—Joe Walsh |
Before email and instant messaging became ubiquitous, voicemail was a popular |
method of electronic messaging. Even though most people now prefer text-based |
messaging systems, voicemail remains an essential component of any PBX. |
Asterisk has a reasonably flexible voicemail system named Comedian Mail.1 Voice‐ |
mail in Asterisk is provided in the dialplan by the app_voicemail.so module. |
A Caveat About Voicemail in Asterisk |
The app_voicemail module is one of the oldest in Asterisk, and it suffers from many |
limitations, especially when compared to other modules that have enjoyed a steady |
evolution. The code itself is something few have the nerve to mess with, and thus new |
features for this module are unlikely to ever appear. You need to understand that |
app_voicemail doesn’t just provide a dialplan application; there are all sorts of things |
that have to happen to ensure this all works, such as storage and file management, |
interaction with the email system of the operating system, time zone awareness, file |
formatting, security, plus a whole basket of various parameters that might need to be |
set. The app_voicemail module does all of that, and as a result it ends up being a sort |
of kludgy subsystem (on traditional PBXs, the voicemail was in fact a completely sep‐ |
arate machine). |
1 This name was a play on words, inspired in part by Nortel’s voicemail system Meridian Mail. Nortel (and |
Meridian Mail) are gone, but Comedian Mail soldiers on. |
129 |
Numerous attempts have been made to re-engineer voicemail, but they’ve all come up |
short. The reasons are simple: the level of work (and therefore cost) required to re- |
engineer it (in such a way as to satisfy the needs of a diverse community), coupled |
with a lack of cultural interest in voicemail technology in general, have (thus far) |
quickly killed any such initiative. |
It’s important to state that Asterisk voicemail works, and works well. You’ll probably |
find it suitable to your needs. If you don’t, the community will suggest that you are |
more than welcome to take a crack at re-engineering it. |
Some of the features of Asterisk’s voicemail system include: |
• Unlimited password-protected voicemail boxes, each containing mailbox sub- |
folders for organizing voicemail |
• Different greetings for busy and unavailable states |
• Default and custom greetings |
• The ability to associate phones with more than one mailbox, and mailboxes with |
more than one phone |
• Email notification of voicemail, with the voicemail optionally attached as an |
audio file |
• Voicemail forwarding and broadcasts |
• Message-waiting indicator (flashing light or stuttered dialtone) on many types of |
phones |
• Company directory of employees, based on voicemail boxes |
We’re now going to take you on a tour of the essential parts of the voicemail configu‐ |
ration file, covering the settings in the general section, the various regional settings |
that are possible, integration of voicemail into your dialplan, and a brief under-the- |
hood look at how Asterisk stores voicemail in the Linux filesystem. |
The voicemail.conf File |
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