no sound in ALE-20H

[mohamedbassuony]

hi
I have an issue with some of the company's phones, ALE-20H. Three external lines have been connected to receive calls outside from the Internet provider.
But I noticed that sometimes I hear the sound of these phones, and then the sound disappears, and then when I disconnect it and turn it back on again, the sound comes back. also, this issue is in internal calls and external.
Knowing that these phones have a hunting group to receive calls on three lines in parallel.

any idea, please

thanks

[frank]

H for hybrid. Are they connected as IP or Digital?