Call history

Instead of having mobile number in call history can it not be possible to show name of caller as in contact list of user.

That would mean you uploading your contacts list to lebara or allowing the app to access your contacts list I doubt they want the data privacy risk.

They already have access to my contacts via their app

Thats not correct the app has no option and android gives no options to enable access to contacts. This is not enabled on android platform.