I think it really depends on what ministry we are talking about here. For example, if you're going to serve as an usher, attitude is very very important. Calling doesn't really matter in this case because ushering isn't really a talent.
However, if someone wanted to be in the drama ministry, calling is very important. Not everyone is able to act, dance, sing, coordinate, express themselves in a way that glorifies God. Attitude is also very important, but if you have a great attitude, but you can't act, then you probably won't make it into the drama ministry.
Just a thought. God Bless!