How many EDGE time slots have you activated in the cell? If you’ve defined only one EDGE TS, then don’t expect the MS to use more than one.
With the GPRS, the system can allocate more TSs if required, but with EDGE, it’s kind of fixed (‘cos you configure a fixed no. of EDGE TSs). Anyway, I use Ericsson, and I’m not sure if it’s the same as ALCATEL, so just check to see how many EDGE TSs you’ve defined in the cell.
Hope this helps.