Real Dictionary

call in
phrasal verb: If you call in somewhere, you make a short visit there.
phrasal verb: If you call someone in, you ask them to come and help you or do something for you.

call in in American English
to call for payment; collect
adjective: of, pertaining to, or featuring such phone calls or conversations
noun: a program in which listeners or viewers phone in comments or questions to the host or a person being interviewed

call in in British English
of or having to do with a radio or TV program during which members of the audience call in by telephone to comment on subjects under discussion, ask questions of guests, etc.
to make (something) the subject of disagreement
to pay a visit, esp a brief or informal one
to summon for help or consultation
noun: a radio programme listeners phone in to
noun: a radio programme which invites its listeners to telephone the presenter

call-in
countable noun: A call-in is a programme on radio or television in which people phone in with questions or opinions and their calls are broadcast.