Attended transfer problem


I am new with asterisk and I have my first problem with the attended call transfer feature.

When a call comes in, i take the call and i would like to transfer it.
So I press the * button (mapped for the attended transfer in features.conf) and the number for the receiving extension.

The receiving extension rings and the call can be taken there.
So far so good.

Now to my problem:
If I hook on the handset BEFORE the receiving extension take the call, the caller from outside will be disconnected and the receiving extension stops ringing.
Shouldn’t the receiving extension keep on ringing until the call is taken?
(as it is with the blind transfer feature)

The incoming line and all of the extensions are POTS, connected on a tdm400p card.

I use asterisk 1.2.4 and zaptel 1.2.3

Hope someone could help me.