System information

ringtimeout = 20
A timeout may be specified in seconds for how long this station will ring before
the call is considered unanswered. There is no timeout set by default.
ringdelay = 5
A ring delay in seconds can be specified for a station. If a delay is specified, this
station will not start ringing until this number of seconds after the call first came
in on this shared line. There is no delay set by default.
hold = private
Hold permissions can be specified for a specific station as well. If this option is set
to private, any trunks put on hold by this station can only be picked back up by
this station. By default, this is set to open.
trunk = line1,ringtimeout=20
A ringtimeout can be applied to calls coming from only a specific trunk.
trunk = line1,ringdelay=5
A ringdelay can also be applied to calls from a specific trunk.
Limitations
While Asterisk makes many things easy, SLA is not one of them. Despite this func-
tionality being intended to emulate simple features, the configuration required to make
it work is fairly complex. Someone who is new to Asterisk and only wants a simple key
system setup will have to learn a lot of complex Asterisk and SIP phone concepts to get
it working.
Another feature that still needs some development work before it will work seamlessly
with SLA is caller ID. At the time that this functionality was written, Asterisk did not
have the appropriate infrastructure in place to be able to update caller ID information
throughout the duration of the call. Based on how this functionality is implemented,
this infrastructure is required to make the display on the phones useful. It does exist as
of Asterisk 1.8 but the SLA applications have not yet been updated to use it. The end
result is that you can either have no caller ID information at all, or you can enable it
and understand that the phone displays are not always going to display correctly as
changes happen throughout the duration of a call.
Another limitation, most relevant to usage of shared extensions, is that transfers do not
work. The main reason is that transfers generally involve putting a call on hold for a
short time. Call hold is processed in a special way with SLA, in that the held call is not
controlled by the phone that initiated the hold. This breaks transfer processing.
In summary, SLA is not necessarily simple to set up, and it comes with some significant
limitations. With that said, if what does exist suits your needs, by all means go for it.
328 | Chapter 14: Device States