In order to setup Asterisk to ring multiple phones from the same dialed extension, you will need to create a phantom extension. I accomplished this by doing the following…
Before we go any further, let’s use the following information as true. The extension we want to have ring in multiple places is extension 100. For sanity’s sake, let’s say we want it to ring in 3 places (regardless of the reason). This means that each phone will need its own extension and auth information according to the sip.conf.
First you need to assign each device (phone), its own extension. Let’s give each device the extension of <ext><n>. Therefore our 3 phones will have the extensions of 1001,1002,1003 respectively. Their entries in the sip.conf will look like this:
 type=peer context=internal username=1001 callerid=Eric Lubow <100> host=dynamic firstname.lastname@example.org call-limit=100 nat=no canreinvite=yes mailbox=100@allstaff disallow=all allow=gsm allow=ulaw astdb=chan2ext/SIP/1001=1001  type=peer context=internal username=1002 callerid=Eric Lubow <100> host=dynamic email@example.com call-limit=100 nat=no canreinvite=yes mailbox=100@allstaff disallow=all allow=gsm allow=ulaw astdb=chan2ext/SIP/1002=1002  type=peer context=internal username=1003 callerid=Eric Lubow <100> host=dynamic firstname.lastname@example.org call-limit=100 nat=no canreinvite=yes mailbox=100@allstaff disallow=all allow=gsm allow=ulaw astdb=chan2ext/SIP/1003=1003
Next, in your extensions.conf, add the entry to ring all the extensions when the phantom extension is dialed. The Dial() command should now look as follows:
exten => 100,1,Dial(SIP/1001&SIP/1002&SIP1003,18)
A nice thing to do to (in order to not confuse the user) is, in your tftp files, ensure that the label on the phone (each phone) is still the actual extension of the phone that one would dial to get to it. Label the phone elsewhere with your REAL extension to keep track of it.