Toshiba Bob,
Every DKT2000 series phone has the exact same hookswitch design. The design of the hookswitch and the circuit path that controls it is flawed. Every one of them that is used with any frequency will go bad. Yes, there were a few lot numbers with bad hookswitches out-of-the-box that Toshiba replaced for free (4-5 years ago??) but they are all long since taken care of. I do not know who told you that those few lots are the only ones with lousy hookswitches (probably Toshiba), but they were dead wrong. The circuit path carries too little voltage and the contact itself is too small.
Sign that your hookswitch is failing: You start dropping calls randomly, especially when transferring or going onto/off of hold.
Sign that your hookswitch is shot: You can only use the phone in speakerphone mode.