Firstly, create a "Custom Definitions" for the parameter the conversion is based on. Go to "Configure" => "Custom Definitions" and click "Create custom dimensions".

1.1. Enter a name such as "Unique_call" (without space).

1.2. The default scope is "Event". Do not change this value.

1.3. In the description, enter "Unique_call".

1.4. Select an event parameter such as "uniqueCall".

Let's set up the "Audience". Go to the "Configure" => "Audiences" section. And click "New audience".

2.1. Then click "Create a custom audience".

2.2. Enter a name for the new audience (for example, "Unique calls"). After that, set up the conditions:

2.2.1. Click "Events" and select call.

2.2.2. Add the "Unique_call" parameter that you created earlier.

2.2.3. For the conditions, select "exactly matches (=)" and set the value: "1".

2.2.4. Then specify the "Audience Trigger" without spaces. For example, "Unique_call".

Save the audience.

The last step is to set up conversion for this audience. Go to "Configure" => "Conversions". And click "New conversion event".

Give the conversion name the same as the audience trigger — "Unique_call" and save.

Conversion events are automatically enabled.