In a bridge architecture scenario, RTCP receiver reports must be enabled in the bridge in order to visualize round-trip time (RTT) values in the callstats.io dashboard. Some bridges may not have receiver reports enabled by default, so please be sure to verify this (see this example for Asterisk).
For a little background on what this means, the Real-time Control Protocol (RTCP) is used by Real-time Protocol (RTP) to report the performance of a media stream. RTCP utilizes Sender Report (SR) and Receiver Report (RR) messages to this end. Senders use the RTCP SRs to assist in synchronizing the media streams, while RTCP RRs are used by the receivers to notify about current loss fraction, jitter, sequence numbers, and to the round-trip time (RTT).
On the other hand, codecs can also play an important role in RTT reporting. In the case of FreeSwitch, OPUS codec has to be enabled in order to get RTT information on the dashboard. This is because OPUS enables RTCP since it needs that information to tune itself.