Balanceo de llamadas en Troncales SIP

El requerimiento es que las troncales tengan el mismo nombre seguido de un número, empezando con el cero. En el ejemplo mis troncales se llaman R0,R1,R2,R3.

[from-internal-custom]

include => dialroundrobin
include => roundrobin

[roundrobin]

exten => _1X.,1,Set(last=$[(${DB(rr/last)}+1)%2]) ;cambiar el valor 2 por la cantidad de troncales en su RR
exten => _1X.,n,Set(DB(rr/last)=${last})
exten => _1X.,n,Dial(SIP/R${last}/${EXTEN}) ;Cambiar R por el nombre de su troncal
exten => _1X.,n,GotoIf(${DIALSTATUS}="BUSY",1,fin)
exten => _1X.,n(fin),Hangup

[dialroundrobin]

exten => _1X.,1,Goto(roundrobin,${EXTEN},1)


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                                                                    PRUEBA EXITOSA
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   -- Executing [1809754@from-internal:1] Goto("SIP/1200-00000024", "roundrobin,1809754,1") in new stack
    -- Goto (roundrobin,1809754,1)
    -- Executing [1809754@roundrobin:1] Set("SIP/1200-00000024", "last=1") in new stack
    -- Executing [1809754@roundrobin:2] Set("SIP/1200-00000024", "DB(rr/last)=1") in new stack
    -- Executing [1809754@roundrobin:3] Dial("SIP/1200-00000024", "SIP/R1/1809754") in new stack
  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
    -- Called SIP/R1/1809754
       > 0x2d0e650 -- Strict RTP learning after remote address set to: 192.168.50.63:16414
    -- SIP/R1-00000025 is making progress passing it to SIP/1200-00000024
  == Spawn extension (roundrobin, 1809754, 3) exited non-zero on 'SIP/1200-00000024'
  == Using SIP RTP TOS bits 184
  == Using SIP RTP CoS mark 5
       > 0x7fc74c008c20 -- Strict RTP learning after remote address set to: 192.168.50.20:9172
    -- Executing [1809754@from-internal:1] Goto("SIP/1200-00000026", "roundrobin,1809754,1") in new stack
    -- Goto (roundrobin,180975,1)
    -- Executing [1809754@roundrobin:1] Set("SIP/1200-00000026", "last=0") in new stack
    -- Executing [1809754@roundrobin:2] Set("SIP/1200-00000026", "DB(rr/last)=0") in new stack
    -- Executing [1809754@roundrobin:3] Dial("SIP/1200-00000026", "SIP/R0/1809754") in new stack