Time-Zone Processing with Asterisk, Part I

At this point, Asterisk has all the data it needs to restrict calls based on the time of day. The Asterisk dial plan can be used to check the time at the remote site; if it is before, say, 8 am or after 10 pm, the phone plays the remote time to the caller and asks whether the extension should ring anyway.

First, Asterisk needs to pick up the phone and compare the current local time to the 8 am to 10 pm window. Asterisk's STRFTIME function converts an epoch into a time of day. By adjusting the current epoch time with the offset value, the STRFTIME function returns a time of day. A call that is too early or too late jumps to code that silences the ringer. When a call's ring is silenced, this example code allows the caller an override:

      
300 => {
     Answer;
     Playback(msg/remote-extension-greeting);
     Set(TZ_OFFSET=${DB(tz/${EXTEN}-TIMESKEW)});
     Set(RMT_EPOCH=$[${EPOCH}+${TZ_OFFSET}*60*60]);
     Set(REMOTE_CLOCK=${STRFTIME(${RMT_EEPOCH},
          Europe/London,%H:%M)});

     if("${REMOTE_CLOCK}" < "08:00") {
          goto 300|too-early;
     };
     if("${REMOTE_CLOCK}" >= "22:00") {
          goto 300|too-late;
     };

     goto 300|normal-ring;

The confirmation code starts by silencing the ring, but allows the user to enable ringing by pressing one. I've allowed any caller the flexibility to override my hours, but this conceivably could be handled by allowing only select callers to be able to override. The “standard” way to set the ring tone is to set the ALERT_INFO channel variable. My ATA is a Sipura, which allows the definition of eight ring cadences. I have defined a cadence called silence, which is a ring that never uses the bell:

too-early:
    NoOp(Too early to ring);
too-late:
    NoOp(Too late to ring);
    Set(__ALERT_INFO=silence);
override-silence:
    Playback(msg/my-remote-time-is);
    SayUnixTime(${RMT_EPOCH},Europe/London,
         A \'digits/at\' IMp);
    Read(CONFIRM,msg/press-1-to-confirm-call,1);
    if (${CONFIRM}=1) {
         Set(__ALERT_INFO=Bellcore-r1);
    };

Ringing the phone is straightforward, because it requires using only the Dial application, and the ring cadence has been set elsewhere. Alternatively, the call can be sent straight to voice mail:

normal-ring:
    Dial(SIP/300,20);
vm-only:
    VoiceMail(umatthew);
Hangup;
};

For enhanced code re-use, the time-of-day checks could be incorporated into a macro that is called as part of every extension.

As a first step, install the GeoLiteCity database from MaxMind, and install the time-zone lookup script into /var/lib/asterisk/agi-bin. To call that script, add the time-zone configuration extension *89 to your dial plan. Every extension that requires time-of-day-specific treatment needs to have its dial plan modified with code similar to that shown in Step 3 of this article.

Then, as a user of the PBX, every time a SIP extension is registered, you need to call *89 to set up the time zone. This need to initiate the configuration process manually is somewhat annoying. Asterisk does provide an interface that can be used to set up the call to the user automatically, which I will describe in a follow-up article.

On a PBX that supports multiple users, several items would benefit from centralized storage. In this example, the time-of-day comparison is coded into the target extension's dial plan. By storing this data in the Asterisk database, it would be possible to let users change their time-of-day schedules without administrator intervention.

Finally, I also have configured my PBX with a “friends and family” override feature that allows selected callers to complete a call even if it normally would be blocked. Callers on the privileged list can request the time of day at my location and are allowed to ring a call through even if it would ordinarily be blocked.