I have a number of 7324 phones connected to a 616 and am having problems programming external numbers into the autodial programmable buttons.

I've got the number into the button by doing the following:

External Autodial:
Feature * 1
Press the button to be programmed
Dial the number, then press the soft button under 'OK'

This works fine. But it's after I program the number that
I can't use it. When I press the button the display says "No free lines". However, if I press one of the green buttons to get a line and then press the programmed number button it works fine. Is this how it's supposed to work or should it automatically allocate a line when I press the programmed number button?

My problem is that the system definitely does have spare lines but the
extension or the system doesn't appear to be able to allocate them.

Does anyone have any ideas where I've programmed the system incorrectly