Mar 17, 2016 · Called the “Security Number”, and is usually the primary account-holder’s 8-digit birthdate: Straight Talk: Phone purchased from ST: Serial number of phone BYOD: Last 15 digits of SIM number: On the website under “Manage My Profile” May also be the last 4 digits of SIM number: T-Mobile Prepaid: 11-digit phone number being transferred ... Visit Crain's Chicago Business for complete business news and analysis including healthcare, real estate, manufacturing, government, sports and more The connection to the mobile network is made by acquiring a SIM card number from the mobile operator and installing it in the gateway. Typically, direct-to-mobile gateway appliances are used for hundreds to thousands of text messages per month. More modern appliances now offer the capability of send up to 100,000 messages each day. Straight Talk has a wide selection of phones, including a few budget devices that are free with plan purchase. If you can't pay the full price of the phone, up to 24-month financing is Straight Talk is a popular prepaid carrier because it's easy to keep your phone, number and the network that you love.