Difference in ring back tone , who can explain?

I have two freepbx systems configured almost the same :

SPA504G phone—freepbx—cisco1750isdngw—isdn_network

SPA504G phone—freepbx—mediatrix4400isdngw—isdn_network

The trunk between freepbx and cisco is configured with NO registration:
host=172.16.217.110
type=peer
insecure=port,invite

The trunk between freepbx and mediatrix is configured with registration (mediatrix register to freepbx trunk):
host=dynamic
type=peer
secret=mypsswd
port=5061
deny=all
allow=alaw,ulaw
nat=no
qualify=yes
canreinvite=no

Same outbound rules in both pbx

When I make an ISDN call, the first system gives me the ringback tone coming from ISDN network (italian tone),
the second system gives back the ISDN ringback tone for a fraction of second, then the ringback tone configured on cisco phone (north america)

Could you explain me why is this and eventually how to get the second system to get the tone from ISDN network ???

Thank you very much