On the sidebar, select "Numbers". On the Numbers page, select the blue "Buy a Number" button on the top right. This should open a new dialog.
Buy a Number Dialog
For the Country field, you may want to pick a country where most of your contacts will be located. Select the number capabilities that you need. However, be sure that your number will be able to support SMS.
Finally, select a phone number from the dropdown menu and the duration for which you want to pay now.
Press the blue "Next" button when you have selected your desired phone number and confirm your purchase.
Your virtual phone number must have SMS compatibility to work.