Can only answer calls from specific extensions

We encountered a special issue today. We currently have 6 extensions on our PBX, and set it to ring all when people calls our DID. However only 3 of the 6 extension have sound when answered. The rest is silence on pick up. We have double checked that the issue is not with the actual phone, when we swap the phone arouund, it's still the specific extension that can answer, the rest is silent on pick up. All settings except login is different on the phones.