speech frame loss compensation

Hi.

Right now in osmobts when sending/receiving frames with osmo_rtp_* it's assumed that no frame is lost and timestamp is always advanced in 160ms steps. In practice (especially when DTX is in place) frames do get lost so we have to adjust the step to compensate.

I've tried to do it as follows: - store frame number of last used frame on receiving/sending - check how far current frame number from old one - convert frame number delta to ms: each frame is 4.615 ms long - if delta in ms is bigger than default 160 than use it

However the result sound not much better than using hardcoded value which suggest that I might be doing FN -> ms conversion (or smth else) wrong. Any ideas/advices?