"maybe" is an adverb because it qualifies / describes the verb form "call". I might say e.g. "Maybe call me if you want to discuss the issue further". I was speaking about a new thing with someone. They needed time to think more about the issue and so I offered them the option of calling me. There is a song I heard recently with the line "Call me, maybe" http://www.azlyrics.com/lyrics/carlyraejepsen/callmemaybe.html Checking the lyrics, the theme is romance and the singer is inviting someone to call her if they would like to progress their relationship.

In this context I think the word 'maybe' is being used as an invitation to be called, it could be seen as a little provocative, especially if they really want you to call.

I do not have the context, but it is possible the speaker is expressing the idea that his or her opinion is not firm.