Some jobs can get along, but not others

[DjeSan]

Hello
I'm having problems with a digital telephone which, when it calls me, I can hear it very well, but when it calls certain numbers, I can't hear it at all.
I tried deleting the number he was trying to reach, then I recreated it and asked him to try again, but nothing changes, same thing, we manage to hear each other, but on certain numbers, same thing, I reset the card on which he is rstcpl but still nothing, anyone have an idea?

[frank]

hi
please give more details.
analog phones? digital? ip?
Are those internal calls? External calls?
Is there a network involved? Are all the calls on the same shelve?