Device or ressource Busy - Dial-Problematik

exten => _X.,1,Dial(SIP/70&SIP/50&SIP/20&ZAP/2,15,tTwW)
exten => _X.,2,Dial(SIP/70&SIP/50&SIP/20&SIP/60&ZAP/2,15,tTwW)

    -- Executing Dial("mISDN/1-1", "SIP/70&SIP/50&SIP/20&ZAP/2|15|tTwW") in new stack
    -- Called 70
    -- Called 50
    -- Called 20
    -- Called 2
    -- Zap/2-1 is ringing
    -- SIP/70-c3b7 is ringing
    -- SIP/50-e846 is ringing
    -- SIP/20-f202 is ringing
    -- SIP/20-f202 is ringing
    -- Zap/2-1 is ringing
    -- SIP/20-f202 is ringing
    -- SIP/20-f202 is ringing
    -- Zap/2-1 is ringing
    -- SIP/20-f202 is ringing
    -- Zap/2-1 is ringing
    -- SIP/20-f202 is ringing
    -- Nobody picked up in 15000 ms
[B]    -- Hungup 'Zap/2-1'[/B]
    -- Executing Dial("mISDN/1-1","SIP/70&SIP/50&SIP/20&SIP/60&ZAP/2|15|tTwW") in new stack 
    -- Called 70
    -- Called 50
    -- Called 20
    -- Called 60
[B]Mar 31 19:41:24 WARNING[10892]: chan_zap.c:1829 zt_call: Unable to ring phone: Device or resource busy
    -- Hungup 'Zap/2-1'[/B]

  exten => _900XXXXX,1,Wait(${EXTEN:4:2})
 exten => _900XXXXX,2,Dial(SIP/${EXTEN:5})

Then in the incoming context you could use the Local channel construct and dial:

 exten => s,1,Dial(SIP/200&SIP/201&LOCAL/90015300&LOCAL/90015301)

In the above example 200 and 201 extension will ring immediately, and 300 and 301 will start ringing after 15 seconds. After to 900 the first two digits are for the delay before start ringing and the last three are the extension that should be called. The trick here is that instead of simply using two Dial statements after each other we make sure we ring each phone only once; that prevents a) potential problems with SIP devices needing some wrap up time before being able to take another call, and b) avoids too many "missed call" entires in the phone's interface.