An SMS Plan is needed to add messaging credits to your account. Credits are the virtual currency used to send SMS. Therefore 1 credit is needed to send 1 SMS and so on.
A $25 plan for example is equated to a certain amount of credits depending on the destination of your messages.
More than $500 please finish the registration proccess, after login in the platform: Actions/Remaining Balance/Buy credits Now and Contact Us option
Please enter the code shown in the picture.