Giving your credit card information over the phone is risky; if you're in a public place, anyone can eavesdrop on your conversation and write down your credit card information. The fewer people who hear your personal financial information, the less likely it is to be stolen.
That's why we encourage you to use the dashboard to change any billing information. When you add or change a credit card using the dashboard, it passes through our servers and goes straight to the payment processor. Your credit card information is never kept on our servers, so if there is ever a security breach on our servers, your information is safe.