Hi my name is ***** ***** you for contacting Just Answer, I will assist with your query today.
Firstly you may find it useful to download a copy of the missing manual from here, should you need to make changes in the future to your phone.
On page 64, there is a Q&A section and one question relates to CLI (Caller Line Identification):
It states that CLI to be displayed this need to be enabled by your telephone provider, assuming this is a residential phone line, in the first instance I would contact your telephone line provider and ask them to confirm they have CLI enabled. On reading page 21 of the above manual this confirm the phone will display the Caller ID (CLI) if this is provided by the phone line provider, it also provides a reason for why you may see other message in place of the number such as 'Withheld', 'External' or 'Unavailable'.