I think I had a similar problem many years ago at a house. I don't remember all the details, but I do remember that the owner could not talk to the caller.
Just an FYI, when you select a line to make a call, you are muted util you dial a phone number. There may be a case where the phone system is thinking that you are trying to make an outgoing call instead of pickup up an incoming call.
At that time I tried to do an Immediate cut-though on the CO trunk. This worked but it completely messed up the redial function.
Another option is to create a new public numbering plan to use a single digit for an outgoing number. Next time the caller can not hear you , you can press the single digit and the Toshiba will assume that you dialed a valid number and un-mute your phone. You should be able to use a code that you would normally not dial, such as the * or #.
You can try this after you made sure that you don't have a handset, hook-switch, or cord issue.
The issue you are talking about is due to the hook switch causing the incoming call to be hung up on when trying to answer the call.
Because the call came in on a PSTN CO line you could press the line key to get the caller back but as you had not dialled anything the speech path would not be opened, therefore if you dialled something like a single digit and it matched the dialling plan then the speech would be opened up.
I would be still looking at the cause of the issue as probably a faulty phone.
Have you changed the phone yet?
