Getting new numbers is the simplest option and is recommended to avoid any hiccups and downtimes. However, if you need to keep your current numbers for any specific business reason (for example, customer support, brand equity), contact Sinch who can help you port them to Sinch for voice and/or messaging, or text-enable them with Sinch via our Hosted Messaging process. It is possible to provision your existing landlines and long codes as 10DLC.
Note: If your numbers are already text-enabled with a different provider, it can take some time to migrate messaging service to Sinch (it depends on the provider who is currently providing messaging service). Sometimes the losing provider may reject the text-enablement request. Porting your number (Voice+SMS) is usually more straightforward. See the article Porting and Hosting Numbers with Sinch for more info.