Source: iMore In macOS Big Sur, you have been able to use the real-time text (RTT) protocol for conversational text calls. In the United States, you can make and receive RTT calls on Macs introduced in 2012 and later, except for the Mac Pro. With RTT, you can transmit conversational text as it is typed or created. The newest technologies that make this possible include instant messaging, Text over IP, and IP-relay. Here's how to do it. To use RTT on Mac, you much have macOS Mojave 10.14.2 or later. It would help if you also had an iPhone with a carrier plan from AT&T, T-Mobile, or Verizon. Standard voice call rates apply.