Problem when forwarding fax to an extension

I’ve an external line. I have created an extension and with follow me component, I said to forward all calls to that extension to external number XXX.
So when a fax arrive, I pick up and transfer to extension 55.
The problem is that the fax doesn’t arrive because after 30 seconds more or less, the communication is dropped. I don’t know if from asterisk or from ATA HT503 grandstream.
What can I do?