It's an interesting question. Given that in the movie, Obi-Wan first attempted to contact the Jedi Council, and only when he found they were out of range did he try contacting Anakin, why did he simply record a message and not get R2 to relay his call directly to the Jedi Council? After all, we see in a later scene that the ship on Tatooine has a good line to the Jedi Council. Could Obi-Wan, one of the wisest Jedi knights ever, have not thought of this trivially simple expedient?
Oh wait, this is the same Obi-Wan who "hid" Darth Vader's son on his home planet, with his only living relatives. Okay, maybe it all fits together.
Or was there another reason?
Obi-Wan: R2, take a message. I'm on...
Obi-Wan: What's the name of this planet?
Obi-Wan: ... Geonosis.
GM: It is?
Padmé: Yeah: geo, nosis. It means "the process of becoming Earth-like". Nice name for a young volcanic world.
GM: Right. Totally what I intended.
Obi-Wan: Actually, R2, patch me through to the Jedi Council. I need to speak directly to them.
Obi-Wan: I'm sure a droid of your "superior capabilities" can manage that.
R2-D2: What was that? <krshkhk>
R2-D2: ... your signal... <krshkhk>
R2-D2: ... going through a tunnel... <krshrshrshkhk>
R2-D2: ...aking up... <ksshhhh>