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In the VoIP world, many companies have started to offer you what they call hybrid PBX systems. In general terms, a hybrid system is one that uses VoIP and PSTN (traditional telephony) at the same times. The strategy is not always important, some will suggest a fallback strategy others will suggest a least cost route strategy. Whatever the strategy is, it is just dial-plan configuration.

I will talk how a customer with a little of my assistance was able to configure his ATA Cisco/SPA8800-6.1.7(GW) to be able to route calls through the PSTN network without acquiring those very expensive and not less difficult to configure SFO'es. I won't talk about how to configure the ATA, in this specific case he must wait 1 second, then dial the number and it must end with a # for at least 150 ms. Note that this is a very specific configuration of this particular ATA, you should read the manual to know how to configure your PSTN dial-out sequence in your own device.

The following dial plan does this magic:

<extension name="PSTN">
    <condition field="destination_number" expression="^\*\*(\d+)$">
        <action application="export" data="nolocal:execute_on_answer=send_dtmf W$1#@150"/>
        <action application="transfer" data="999 XML mycontext"/>

If you think a little, you can use this to take advantage of the dry-loops. Almost every dry-loop supports 911.

Good luck!

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