IAX2 stopped sounds

I have two Asterisk servers conected with IAX. Calls between SIP’s and calls by PSTN conected on server1 are stablished between both servers without problems. But when I try to receive a fax on server2 I receive the following lines on server1:
IAX2/ast11-16390 stopped sounds
IAX2/ast11-16391 answered SIP/387-089e1508
And it seems that I can hear no sound and the Server2 hangup the conection, but the server1 even receives this hangup.

Why am I receiving this ‘stopped sounds’ and what does it mean?
At other calls I don’t receive this message.

Can somebody help me?