There are 2 issues here.
1. I believe that due to the size of the latest software, the web interface was completely removed/disabled.
2. Even before that, if you used the web interface in the A800 before and you did not have problems. Consider yourself very lucky as the web interface has not really worked correctly since the beginning.
You should start using and getting used to telnet/cli as this is the only way to really configure the A800.
If you want to disable/enable channels in the pstn port, you do the following from telnet;
config pstn 1
dis 1 x (where x = channel number or a for all)
ena 1 x (where x = channel number or a for all)
print (to check your setting)