3CX has Company and Personal contacts. You will see this on your iPhone app and the Webclient.
Personal contacts are visible to the extension user only and Company contacts are available to all extensions.
So, where you have customers that you want their name to pop up when then call, simply add them to the company phone book. Use the web client or the iPhone app.
On the web client, click on Contacts then Company then the "+" symbol.
When you add contacts they will then be available to everyone and you will get the name appear instead of the number.
Another option you have is to import via CSV. We can provide a CSV file template.
There is also Office 365 cloud to cloud integration which will sync contacts per user so contacts people have in their outlook contacts will sync to their personal contacts.