Yeah that's
not how it works. You can't just "put the complicated antenna on the satellite, instead of on the phone" and declare this problem solved. If you believe that I have a bridge to sell you.
I'm declaring shenanigans right now on Elon's pitch but what else is new. It makes me sad that this thread is full of (probably) SWEs in a complete froth who took this at face value with little understanding of how antennas work. No, your existing phone will not be able to work on this service, full stop.
The issue is not getting signal from the satellite to your phone. That's the easy part - after all, your phone receives GPS signals. Satellites have powerful transmitters and high-gain antennas. Even GPS which is considered extremely low-power has a TX power of around 20-25 watts and EIRP is probably an order of magnitude higher after the antenna (probably enough to turn your eyeballs into Easter eggs if you were standing in front of it). The issue is going the other way. For messaging to work, your phone needs a two-way connection. Your current iPhone/Android handset is simply not designed for satellite communication.
"But muh Garmin inReach" ..... have you stared at one and wondered "why can't my iPhone do this"? First, the antenna is the size of a thumb, it looks like a helical antenna to me, which would make sense given that most satellite communications uses something called "circular polarization" to combat things like the Faraday effect. This requires an antenna specifically designed for it. Your iPhone doesn't have one, which means more signal loss. Iridium phones look similar - very Michael Douglas in Wall Street industrial design. Satellite phones and communicators are designed to direct most of the energy upwards towards the sky. Your existing phone does not so you are losing most of the energy in an omnidirectional pattern. Even things like 406 MHz emergency beacons send extremely short message bursts - just a few bytes - the link budget needed to sustain an IP connection (uplink) with an existing handset just isn't there.
So I'm expecting Elon to deliver on the magical complicated-super-antenna that works with your existing mobile handset right around the same time he gives us his FSD Cybertruck. Which is to say, never.