RANG-193: Explain that default prompt voices are selected based on the campaign country

Author: fdisperati-invoca

source/api_documentation/network_integration/prompt_voice/index.rst

@@ -13,6 +13,8 @@ Set Prompt Voice
 
 To use a particular prompt voice for your campaign, set ``"tts_voice_talent_id"`` on the IVR. See the tables in the **Conditions** section below to determine which prompt voice to use. (Make sure to reference *only* the table that corresponds to the language of your campaign.)
 
+If no voice is provided, a default voice will be selected based on the Campaign Country.
+
 POST
 
 ``https://invoca.net/api/@@CAMPAIGN_FEATURES_API_VERSION/<network_id>/advertisers/<advertiser_id_from_network>/advertiser_campaigns/<advertiser_campaign_id_from_network>.json``