Secure feature access from an off-PBX telephone

A security plan for telephone clients allows the client to authenticate itself by sending a coded sequence of DTMF digits to a PBX at the beginning of each call. When the idle client telephone receives a call, it sends a coded DTMF sequence to the PBX. If the client makes a feature invoking call to the PBX while active on the first call, the client will be prompted for the security sequence. The client must send the same sequence of DTMF digits as for the first call. The first call is authenticated by virtue of being a call from the PBX to the telephone. The second call is authenticated by sending a DTMF sequence that matches the one sent with the first call. A call that fails authentication is dropped. The security plan for clients constrains the client to using a callback to originate a call from an idle telephone.