If you are worried, you are 担心

or if you are not worried, you are 不担心

example using 放心：

It was reassuring to hear your voice

听到你声音让我放心。

You are wrong, sorry.

放心 is a verb meaning to assure or to put X's mind at rest.

For example you can say to someone 放心吧, this is like saying 'do not worry', or 请你放心, please be assured.

If you say 我不放心 you are saying you are still worried, no one has put your mind at ease. 

If you want to say you are not worried about something you would say something like 我放心了. This means you are no longer worried. 

There is the difference between 放心 and 担心。

放心 is someboy feel safety and relax with something or somebody.

For example：

你办事，我放心。

I am relieved for your working.

担心 is somebody worry about sth or sb.

For example:

外面雨很大，我担心回不了家。

I am worry about for the heavy rain.

example:1放心，没事的

2我放心不下