BLF / Asterisk 1.6.2 / FreePBX 2.7 / Grandstream GXP 2000 -> External Call Problem


I have the following components:
BLF / Asterisk 1.6.2 / FreePBX 2.7 / Grandstream GXP 2000 / Ubuntu 10.4 Server 32Bit

I have configured the 7 additional fields in the GXP 2000s with SIP Extensions.

Now, the basic BLF (Green on Free, Red on Busy, blinking on Ringing) works fine.
If I call internally to another SIP Phone, I can pick it up too fine with a touch on the Blinking button from a 3rd phone.
But, calls from extern I can not pick up! Instead the phone where I try to pickup the call just tries to create a second call to the ringing phone, but If I enter **{EXT} on the phone (instead of pushing the BLF for this extension) the call is picked up.

I hope this was clear:
Internall ringing calls can be picked up from a third phone with the touch of the BLF button.
Externall ringing calls CAN NOT be picked up with the BLF button, rather the phone tries to create a new call to this phone, pickup WORKS with **{EXT}.

I am sure its only a small thing, but which?

Incoming call szenario:

LINE X (4 ISDN Lines)
Time Condition
-> (WorkingHours) IVR Welcome Message + Dial a Ring Group
-> (OffHours) Dial a Ring Group for 10 seconds (for pickup possibility) -> IVR with Night Mode Message -> Hangup

The Ring Group contains 3 Extensions (2 SIP Phones, 1 SPA8000 connected analog DECT Mobile phone)

What I want is that an incoming Call on these 3 Extensions could be picked up from the GXP2000s via the BLF button.

thanks for any help,
best regards