ZTE MF286R voice functionality

Hi!
I have OpenWrt installed on my MF286R device and I am very happy with this decision. Recently, however, I wanted to install a landline phone using the phone port on the back of the router. I only get a busy signal when the phone tries to reach the dialed number. I know that the device page on the OpenWrt wiki says that the voice function is not usable, but I read on the forum that it works, but is managed by the modem itself... So it could also be a problem with my contract.

Does anyone know if the phone ports on the back of MF286R work after installing OpenWrt?

Thank you in advance...