Basically, ----なおす makes a compound verb attached to other verbs' stem of -masu form. If means to do something again all over because the previous try ended up with error. 書きなおす→書き（ます）＋なおす to write again because there were errors かけなおす→かけ（ます）＋なおす　to call again because the calling was not successful Pay attention to the difference of vowels. They make the meaning of verb totally different.

書きなおす。・・・は、間違った文字を書き直す。と使います。 かけなおす。・・・は、相手が電話中や留守の時に、電話をかけなおす。と使います。 全く意味が違います。