The support guy may be correct depending on the scenario-
When you have two line keys, you have set two independent instances on the phone, that when used in conjunction with the UCM extension settings of call waiting, which equates to allow multiple calls, each line is available to take and make calls. SO, if the 1st line is engaged and a second call does come in, then it will ring the phone.
If you are of a notion to only wanting to deal with one call at a time, then simply disable call waiting in the extension settings of the UCM. You will still have the 2 line keys to work with, but the UCM will only deliver the single call and then use whatever rules you have set or VM for the second call.
If you want the option, then you need to determine where you want the function to reside, at the phone (local call features) or at the UCM. The forward will need a target and you also need to consider that some softkeys are only valid for the active call, not the ringing call and is usually a condition that is preset, not dynamically used.
Take a look at the admin manual, the user manual and the UCM and play around with the settings, but I have a feeling that the expectation is that when the second call comes in, one can hit the forward key, then enter the target and press send and cause the call to go elsewhere in the background and that I do not believe is possible.