So I've been digging into this further.
The session variable is blank when the form receives it.
So, I started to look at where the browse sets the session variable.
In the routine "Call Clicked" it sets the session variable to the value of p_web.GetValue('cus:custrefnum')
I sent the value of p_web.GetValue('cus:custrefnum') to debugview and its blank...
Hmm..
So how is the value initially set? I added 'cus:custrefnum' as a column in the browse and it's had no effect....