Ovh call forwarding false DTMF detected

Hi all,
I have two ovh lines ( Line1 and Line2) and one line (Line3) with Free in France.
When line 2 receives a call, asterisk launches a php script that runs an ivr with menus (menu1) and sub menus (menu2), each menu taking dtmf inputs through the background command called from agi_exec (within phpagi library) .
If I call directly line 2, all works fine.
If I call line 1(which is transfered to line 2), if I choose 1 in menu1, it skips menu2 (though I can hear first half second of sound file of menu 2) acting as if it had received also 1 as dtmf.
If I forward line 3 to line 2, all works fine.
So ovh to ovh seems to be an issue.
Any idea?

anyone any idea where to start looking?