asterisk users - Mar 2008 - Little help with Conference

These is my scenario.

Asterisk 1.4.16
Zaptel    1.4.8

Grandstream BT200
Grandstream GXP2020
Grandstream GXP2000

For some reason the end user ask to configurate son direct access  like 
*01,*02,*03 thru *78.

After they began to use these direct access, I cant place a 3 way 
CONFERENCE.

I remove the direct access, but i dont know if one of them block the 
CONFERNCE.

Do you know if i can make reverse for these???

Thanks

Ruben