I don't want the phone to receive more that 1 call. So if an operator have one ongoing call, a new incoming call will received busy signal. As far as I understand, there is no option to set this behaviour using Web Admin of the phone.
If you are using a Cisco PBX, you can configure this directly on the system. the "ephone-dn" config section is used for configuring phone extensions, if you make the primary extension of the phone a single-line extension, as in "ephone-dn 1" instead of "ephone-dn 1 dual-line", the phone will only take one call, and the call-forward busy handler will be used for additional calls.
If you are using the SIP firmware for the phone and not using a Cisco PBX, this is not configurable directly on the phone. If you are connecting the phone directly to a SIP provider, they may be able to accommodate this. If not, you'd have to review the options of the PBX you are using.