In the first, second and sixth SoulCalibur games, the classic numeric input for Ivy's notoriously difficult Summon Suffering throw is 376231A+G (Attack + Guard). This number combination actually holds significance as it appears to be a reference to the phone number for Namco's headquarters from before they were acquired by Bandai, being 03-3756-2311.

Allegedly, the developers used the phone number as the basis for the input as a reaction to an in-joke among Tekken and Soul Edge arcade communities, who would react to degenerate or weird stuff in the games with "gonna call Namco about this". The joke being: "If you can't do the throw, go call Namco about it." However, since this input was brought back in SoulCalibur VI long after the phone number stopped being used by Bandai Namco, this adds an additional (albeit probably unintended) layer to the joke: "Go call Namco about it. Oh wait, you can't."
Contributed by MehDeletingLater
The two Yoshimitsu characters of the Soul series -- The first appearing in SoulCalibur through SoulCalibur IV, the second from SoulCalibur V -- are the ancestors of the Yoshimitsu from the Tekken series.
Contributed by KidDivinegon
In the Korean versions of the first 2 games, the character "Mitsurugi" was replaced with a new character, "Arthur", a Caucasian who fights and dresses like a samurai. It's likely that this change was made due to Korea's fickle relationship with Japan, and that Korea actively discourages samurai imagery and references. Mitsurugi was eventually added to Soul Calibur in Korea, replacing Arthur. Nods to Arthur can be seen in later entries in the series, even outside of Korea. These range from fighting him in certain modes.
Contributed by DidYouKnowGaming