I have exactly the same problem with the H19txt and a couple more.
In the middle a converstion, a little click will occur and my incoming sound is cut off. The first time it occurred I did not realize that the outgoing sound was still connected and so I hung up and called back. It has happened at least 3 other times and I was able to ask the other party to hold while I go directly to the phone itself by closing the boom microphone and avoid losing the call.
Twice, a load buzzing has begun on incoming sound (the caller does not hear it) and I have had to close the boom and go the the handset to continue the call.
Finally, I am used to when wearing the headset with the boom open, to have an audible prompt in my ear of an incoming call. This does happen sometimes, but I have also felt the vibration of an incoming call on my handset with no acknowledgement on the headset. Moving around in a busy environment, the vibration can be missed and I need to rely on the headset to prompt me. Perhaps it had gone into a battery conservation mode? Not a good thing.
I have had an iPhone 4GS and for several months and iPhone 5. I have not had these issues using a H17txt for about a year. That makes me think it is the headset.
I wonder about a firmware update as well. I don't know how it could be applied --but it seems this type thing would be fixed in the firmware.
