I have searched many pages/forums on internet, but couldn't find exactly what is the reason for disconnecting/droping/hangup the call and how it could be fixed. In general the fix ideas are grouped into a few groups:
1. Changing disonnect frequencies on spa400 in accourding to the country's code. See http://www.3amsystems.com/wireline/tone-search.htm
2. "Force proxy" check on SPA9000
3. change "line preferances" on SPA400.
Any other fix idea is very wellcome????
Does anybody else has got the problem? What you have made to fix the issue?