For the ITSP profile in question, try setting RTP->KeepAliveInterval to 5.
Try disabling all codecs except G711U and G711A. For those codecs, make sure that SilenceSuppression is set to no.
If the above does not help, please report: Provider? Fail on both incoming and outgoing calls? If only outoging fails, what destination (country, carrier if known)? In the first few seconds after being placed on silent hold, i.e. before you get disconnected, look at the Call Status page and refresh it a couple of seconds later. In the RTP Packet Count field, has tx stopped changing? Has rx stopped changing? When on silent hold, if you speak every few seconds, does that prevent the disconnect?