Unless things have changed, calls on signal between you and people on your contact list are p2p. This is necessary to have a call experience like what people are used to. The latency of going through a relay makes conversation more difficult. Yes that does mean that someone on the network can see that there is a signal call between the 2 ip addresses (but they wouldn't know the identities of the users) https://signal.org/blog/signal-video-calls/