how do I get a return number for missed calls?

You would find that in the Recent Calls list, unless that number was blocked by the caller. If the caller blocked their Caller ID, you will not see an actual phone number in the Recent Calls list. If it is not available, it will not be displayed.

When you say a fixed line, are you referring to people that are not listed in your contacts? If so, check Settings>Phone>Silence Unknown Callers and turn that off. With that on, callers that are not in your Contacts will not ring through.

Tap on the little "i" in the blue circle at the right of the listing in Recents. If there is no number, it's because the number was blocked.

Scroll down; Phone will be in settings: