I don’t know Nokia BSS architecture to really help but if you have a loss of TRAU Frames (voice) trancoding elements should be able to detect that and raise an alarm or event.
I’ve allready faced that kind of issue and it can be really difficult to investigate. If you cannot identify any common thing between these more or less silent calls it can be become a nightmare. If the issue is really widespread (i mean all BSCs have some site/cells showing the issue) directly ask Nokia support team to help you.
some more question:
– how did you notice this issue (customer complain, Qvoice measurment…)?
– Any software upgrade? (BSS and NSS)
– Are you using DTX (DL and/or UL)?