You can think of a PreTransaction as sending a request to your user to confirm how they want to pay, either by credit/debet card or by phone (SMS), or some other payment option depending on the options available for your account and/or country of your user. After the user has chosen the option and input the necessary payment information, the actual payment Transaction will be performed automatically and user will be informed of the success/failure-status.

A typical scenario:

You POST PreTransaction and specify to whom (mobile phone number) the request should go to and what payment types you accept.
The user receives an SMS with a URL in it. Example text: Please click on this link to pay: <paymentSelectionPageUrl>.
When the user clicks the link, it will open a page in the mobile phone browser where the user can see the invoice and choose payment methods.
Depending of the payment type chosen, they will be re-directed to a page for inputting their card information and a Transaction will be performed towards their card.
After the payment has been completed, the user is re-directed to a page that shows the status of the payment (success or failure)

FLOW DIAGRAMS

PreTransaction basic sequence

Figure 1.1 - PreTransaction - Basic sequence