Sideline does not offer a "read receipt" option. A read receipt shows the sender when the recipient has opened the message.
This functionality is only supported by on-net (on-network) communications. Read receipts would be supported if you're communicating from your Sideline number to another Sideline number (or our Pinger iOS or TextFree app.)
Other examples of on-net messaging would be WhatsApp or iMessage. WhatsApp and iMessage have read receipts and shows when a person is typing because you’re communicating with your contact "on-net" who is also on WhatsApp or iMessage.
If you have any further questions, please submit a request.