A party may amend the party’s pleadings once as a matter of course at any time before a responsive pleading is served or, if the pleading is one to which no responsive pleading is permitted and the action has not been set for trial, the party may so amend it at any time within fifteen (15) days after it is served. Otherwise a party may amend the party’s pleadings only by written consent of the adverse party or by leave of court; and leave shall be freely given when justice so requires. For amendments adding defendants pursuant to Tenn. Code Ann. §20-1-119, however, written consent of the adverse party or leave of court is not required. A party shall plead in response to an amended pleading within the time remaining for response to the original pleading or within fifteen (15) days after service of the amended pleading, whichever period may be longer, unless the court otherwise orders.