Hi @carmengauci,
To see incoming phone call numbers displayed on the phone you need to have the Caller ID (CID) (also can be known as Calling Number Display -CND) facility enabled by your landline service provider.
Depending on your provider this is usually a subscription feature on a landline service.
If you haven’t previously requested the CID facility for your service and would like to have it, contact your landline service provider.
If your phone shows “private number” on ''every'' incoming call then you haven’t got CID enabled.
If your phone is connected to a VOIP service (phone over the internet connection) you may be able to enable the facility yourself by logging into the user interface of the modem and altering the phone settings as a lot of ISP provide the feature for free.