Thank you jozeph, but as I said (not very clearly), when I use
0953XXXXXX@freephonie.net, I have right after a "Forbidden" message. Using only my prefix (0953XXXXXX) in "From User" works well on ATA devices.
After reading the Freephonie.net thread, I think it is a codec problem, but I thought there was a codec negociation and that a message would appear if an error was encountered.
(And when I said Betamax worked well, I didn't see that they charged my calls supposed to be free, because of FUP exceeded... but it's another story).