sla.conf:
[line1]
type=trunk
Device=DAHDI/32
autocontext=line1

[station](!)
type=station
trunk=line1
autocontext=sla_stations

[station1](station)
device=SIP/81

[station2](station)
device=SIP/8

extensions.conf:
[line1]
exten => s,1,SLATrunk(line1)
exten => s,n,Hangup

[sla_stations]
exten => station1,1,SLAStation(station1)
exten => station1_line1,1,SLAStation(station1_line1)

exten => station1_line1,hint,SLA:station1_line1

exten => station2,1,SLAStation(station2)
exten => station2_line1,1,SLAStation(station2_line1)

exten => station2_line1,hint,SLA:station2_line1