Ok guys, Emoci and Martin were right!
The root cause of my problem is that the the incoming call was originated by a Betamax VoIP (voipstunt) and for this reason I got the mentioned busy line from PSTN gateway.
As I were trying to call by a real PSTN line the problem disappeared. All is working fine as Martin said: 'The PSTN numbers are designed to handle calls from fixed line or mobiles. Although they should also work from VoIP originated phones they are not always guaranteed to do so'.
Thanks to everyone
bye
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