A few days ago I was asked to implement a ring group in Asterisk. I thought okay, it's pretty straight forward, I just put all the devices I want to ring in a Dial command and that's it. So I did, but then I was told that that was not entirely ok. They wanted a delay between devices, such that at first only one phone would ring, X seconds later a second device (without the first one stopping) and so on. I thought I could put more Dial commands in a specific order, but then I had the problem that if the first one didn't answer, it would indicate a missed call.

So here is how I finally solved it with help of my colleague Bleek.

[wait-op]
; Ask if the channel is available, if it is
; go to the next step. If it isn't go to no-op
; and skip the delay.
exten = _XX,1,ChanIsAvail(SIP/${EXTEN})
exten = _XX,n,GotoIf($[ "${AVAILCHAN}"="" ]?no-op|s-na|1:3)
; Increment the delay by a value of five.
exten = _XX,n,Set(DB(cross/delay-${key})=$[${DB(cross/delay-${key})}+5])
exten = _XX,n,Wait(${DB(cross/delay-${key})})
exten = _XX,n,Dial(SIP/${EXTEN})
 
[no-op]
; Do nothing
exten = s,1,NoOp(Dummy)
exten = s-na,1,NoOp(Channel is not available)
 
[hotline-0]
; Define a custom name for the caller ID. 
; This was an extra that I did
exten = s,1,Set(CALLERID(name)=hotline ${CALLERID(name)} ${CALLERID(num)})
; Set a key unique for each channel. So id doesn't matter how
; many calls we get, there will always exist just one key per channel
; This way we increase the delay only when we want to.
exten = s,n,Set(__key=${CHANNEL})
; Define the initial delay value on the database. That's even better than
; a global variable. One advantage, pointed out by a collegue of mine, is
; that when the process is over, you can delete the key from the DB.
exten = s,n,Set(DB(cross/delay-${key})=-5)
; Set all the devices as a single variable.
; Note that all of them use the "Local" context
exten = s,n,Set(dg0=Local/91@wait-op)
exten = s,n,Set(dg0=${dg0}&Local/93@wait-op)
exten = s,n,Set(dg0=${dg0}&Local/95@wait-op)
exten = s,n,Set(dg0=${dg0}&Local/20@wait-op)
exten = s,n,Set(dg0=${dg0}&Local/21@wait-op)
exten = s,n,Set(dg0=${dg0}&Local/50@wait-op)
exten = s,n,Set(dg0=${dg0}&Local/22@wait-op)
exten = s,n,Dial(${dg0}|80)
; Manage the voicemail with a macro
exten = s,n,Macro(hotline-voicemail|${DIALSTATUS}|0)
; Delete the keys at hangup
exten = h,1,NoOp(DB_DELETE(cross/inc-${key})
exten = h,n,Hangup
 
[macro-hotline-voicemail]
; ${ARG1} Dialstatus
; ${ARG2} Whose voicemail?
exten = s,1,Set(CHANNEL(language)=de)
exten = s,n,Goto(s-${ARG1},1)
exten = s-BUSY,1,Voicemail(${ARG2},b)
exten = s-NOANSWER,1,Voicemail(${ARG2},u)
exten = s-CONGESTION,1,Voicemail(${ARG2},b)
exten = s-CHANUNAVAIL,1,Voicemail(${ARG2},u)

I know it's perhaps not the best approach. We didn't want to use queues, because our peers aren't agents, and, though it can be optimised, it works now :)

Hope it helps somebody,

Happy coding.