While it's never stated explicitly in the episode, we can make some plausible guesses:
Bail Organa: Bail is definitely aware of and able to contact Kenobi. The way he referred to him in Rogue One, though, suggests they are not in regular contact.
Yoda: Yoda has been in recent, direct contact with Ezra through the Force; he and Yoda have also been training with Qui-Gon. There's no reason to believe that they could not be in contact with one another.
The Grapevine: Kenobi isn't totally isolated on Tatooine; certainly he knows enough to know that Mos Eisley is a wretched hive of scum and villainy, and he's sufficiently in touch with the world that he can rejoin it with ease. Ezra hasn't exactly been keeping a low profile - there's every chance that Kenobi could have heard Ezra's broadcast, or at least have heard about it.