Picking up second call Problem

[LARRYFRANK20]

I am unable to pick up a second incoming call from a phone which is engaged.

I have all outside calls going into account 1.
The other 5 accounts are used for internal call transfers.
When a second call comes in to line 1 while line one is in use, the red light flashes, and the caller ID scrolls across the screen with information on the second call. Caller 1 can be put on hold, but it is not possible to connect to the second incoming call.
( when on hold line one flashes, but when the flashing line button is pressed, The call just takes the first caller off of hold.
How can I connect to the second call coming in .

Thank you
Larry Frank