The below image shows the fields in the SMS action. The SMS message is written in the Message field.

Ensure that the SMS Msisdn field hosts a variable path: InputData.msisdn.ToString()
This msisdn can we swapped out for 'phone' or 'Phone' or 'customerPhone' depending on the field hosting the customers cellphone number in the payloadExample of the syntax for the Message:
'Dear '+InputData.title.ToString()+' '+InputData.customerName.ToString()+', Your debit order for your OnAir Account was not successful. Call 0861001970 to re-activate your debit order details. In the interim please pay your outstanding amount of R'+InputData.billedAmount.ToString()+' by making an immediate payment today by clicking on the SnapScan link: '+Url1.url.ToString()+' or pay via Banking Details: ABSA Acc Number 4086019863; Branch 632005. Payment will avoid a negative listing on your credit profile. Ref: '+InputData.policyNo.ToString()+'
So to break it down:
The message must have quotations marks around it
'Hello, How are you?'
If you add a variable, add it within quotation marks and +'s on either side
'Hello '+InputData.customerName.ToString()+', How are you?'
And you can keep adding variables or content until you are happy with your SMS.
'Hello '+InputData.customerName.ToString()+', How are you? Check your policy by clicking '+InputData.policyNo.ToString()+' Have a good day. '
Keep in mind that 1 SMS is equal to 160 characters, going over this limit will split the message into 2 SMS's. Note: Spaces, commas and punctuation marks must be encapsulated in the quotation marks too.
If you have used/mispelt a field name within a variable path then the action will not be triggered to the customer. Ensure you have used the correct naming convention.