Asterisk 1.4 gxp 2000 pickup

hy, my system:
asterisk 1.4.15 freepbx 2.3.1 on centos5!

i’ve problems with picking up external calls with the gxp 2000 grandstream.
if i call from extension 105 to 106 it’s no problem to pick the call up on the reception gxp 200 by blf.
but if i call from outside mobile phone number to extension 106 the pickup ist not possible! why?
in both cases the grandstream dials **106 to asterisk or not?

thank you in advance