It needs to be the first column in your data, and it only needs to have numbers in it.
When you upload the file, you will be presented with this screen:
It shows you the number of valid and invalid contacts which you can then use to edit your data. If your data contains duplicates, the secondary entries will be marked as invalid at this point.
Here, you can also change your Sender ID using the ‘From’ field, and you can enter your message using either a template or the free type ‘Message’ field.
You can choose between SMS and Voice messages using the tick fields, and schedule the messages using the ‘Send later’ button.
You can see how many message credits the send will use. Remember, if a message is more than one part, you will be charged more than one credit for it.