[Freeswitch-users] voice dropping out

Hello, I have had reports when talking from a sflphone SIP phone to a Gigaset C610 phone, my voice keeps dropping out, As it the other end can't here complete words I say. If however I test with the freeswitch echo service, it works fine, Similar if I tell sflphone to record, it records everything fine. So it appears to be a transport problem, as opposed to a microphone pick up issue. Similarly, the Gigaset C610 is known to be good, and works fine with other sources. freeswitch shows PCMU codec is being used for both legs of the call. (I intended to use PCMA, as from memory that is more "standard" in Australia but I am doubtful that is the problem) How do I debug? Thanks