[downstairs]
include => sipdownstairs

[upstairs]
include => sipupstairs

[sipdownstairs]
exten => _xxx,1,Dial(SIP/${EXTEN},20)
exten => _xxx,n,Voicemail(${EXTEN}@downstairs)
exten => _xxx,n,Hangup

[sipupstairs]
exten => 124,1,Dial(SIP/124@downstairs,20)
exten => 124,n,Voicemail(124@downstairs)
exten => 124,n,Hangup
exten => _xxx,1,Dial(SIP/${EXTEN},20)
exten => _xxx,n,Voicemail(${EXTEN}@upstairs)
exten => _xxx,n,Hangup