External transfer with problems with GXP1625 phone



I have a problem with the External Transfer with phone GXP1625.
From the same phone I transfer an external call to another internal one both with announcement and blind regularly.
The problem arises if an external conversation tries to transfer it to an external number, both with announcement and in blind mode the conversation falls.
It is not a trunk problem as I have 8 channels available and I can do more than 2 simultaneous conversations.
Any ideas?