1.- GOS
A definition is the amount of calls that could have problems to get a free voice channel, is used mainly for trunks, for example GOS = 0.01 mean 1 call per each 100 could have problems to get free trunks.
2.- SLA
Service level agreement, is the time your service will be up, normally it is beyond 97.00 % (includes, acces, transmision, platform service etc), also is used to apply to the service provider some charge in case the service is below of the SLA signed.
QOS?
Sorry, no clear idea…