I'm sorry we did not finished our conversation, anyway, There was no mention about how to hold a call on the manual. But I saw that the unit can switch active phone lines if you subscribed to two phone lines.
and here is the instruction:
change active line
If you have more than one phone number, you
can switch from one line to another.
Find it: s >mPhone Status > Active Line
page 34 of this manual
This is the only way to switch caller. By switching lines. if you have two lines.But the other caller would be dropped and not on hold.
Another thing to look at is if your service provider provides the call waiting feature.
If you have call waiting, your line would still ring on the caller even if you are busy on a call. And your unit will ring and you can put the first caller on hold to answer the second caller.
But if your provider has no feature like this, then there will be no call waiting and the second caller would receive a busy tone to indicate the the phone being called is being used.
So if you have call waiting, normally what you can try is press again the green call button if you hear the unit ringing and the first caller could be placed on hold and the second caller would be activated.