Recently I setup the API connector to POST an SMS messages via a local provider.
In general, to create a line break within an SMS, common characters may include :
\n
%0a
\r\n
I had to use \n to create a line break, but it was not working. The raw response would show duplicate characters.
I filed a bug report, I was advised that the issue may be a result of a
…slight difference in how we handle these key value pairs & raw body json in the API Connector.
As such, we may consider a feature request to let users choose, wherever they enter literal values in an input, whether to treat them as literal values or unescaped values.
The workaround that was suggested to me was to use the boolean :formatted as text modifier, which gives you multi-line inputs, and insert two blank lines within that. Accomplishing a line-break without the need of special characters.
Thank you to @ alessandro , @ dazhane , @neema and everyone else on the support team. 
I received responses within hours, unbelievable. Thank you!