Asterix and Avaya 9620

i’m new member of this forum.
I’ve recently buy an Avaya 9620, and i have installed it on my voip network.
The problem is that the telephone doesn’t do always the first call, emits sound similar at busy user.
I have a problem like this post viewtopic.php?p=180217 , but the firmware 2.6.7 is no longer present on the Avaya site.
Where i download it?
thanks for help.