As akefa said according to the specification of GSM, it is not possible to get information above
the real speech quality, as perceived by the subscribers. It is only possible
to estimate the subscriber perceived speech quality.
But in R8 Ericsson system, speech quality supervision was introduced which gives statistics based on
SQI samples reported to the BSC.
Also I should remind you in TEMS, measuring SQI is included by all necessary factors except Speech Codec.I think this measured SQI is good to rely on.