[Freeswitch-users] Remote audio not heard

[Michael]

While calling certain destination phone numbers (via freeswitch-1.4.7,
and flowroute),

often,

on the calling devices (SPA942, SPA942, GrandStream, old phone on
SPA3102 ATA),

no ringing is heard and,

though the destination has answered,

no audio returns from the destination

(although destination can hear audio from caller).

The same destination numbers sometimes work correctly and sometimes fail
as above.

Logs show normal activity, meaning no difference between normal and fail.

Exact same thing happened on freeswitch-1.2.something, 1.4.6, and now
1.4.7.  Exact same thing happens across wildly different calling phones,
across many different destination numbers.  Just about the only thing
that has not been changed, updated, etc. is flowroute as the carrier.
Could it be a problem at the flowroute end?  Anyone else having similar
experiences?

Any ideas on where to begin?

TIA, Michael