iPhones use Globalstar, which theoretically supports voice bitrates of (I believe) 9.6 kbps, although only using dedicated satphones with large, external antennas.
Apple's current solution requires several seconds to transmit a location stamp of only a handful of bytes, so I think we're some either iPhone or satellite upgrades away from real-time voice communication over that.
Starlink has demonstrated a direct-to-device video call already, though, so we seem to be quickly approaching that point! My strong suspicion is that Apple has bigger plans for Globalstar than just text messaging.
Starlink is in a better position as their satellites are in a low earth orbit - 30 times closer than geostationary. It correlates to 1000 times (30dB) stronger signal on both sides.
Globalstar is LEO as well, although a bit higher (~1400 km) than Iridium (~780 km) and Starlink (below Iridium; various altitudes). In terms of SNR, they're very comparable.
Newer GEO direct-to-device satellites also have huge reflectors and often much higher transmit power levels that can compensate for the greater distance somewhat. Terrestar and Thuraya have had quite small phones available since the late 2000s already, and they're both (large) GEO.
Iridium and Globalstar aren't geostationary. They are LEO not much higher than Starlink.
Starlink is doing direct-to-cell. Talking to existing phones requires a large antenna. The bandwidth for each device is slow, not enough for mobile data, but better than Iridium. I think they recently showed off voice calls.
Apple's current solution requires several seconds to transmit a location stamp of only a handful of bytes, so I think we're some either iPhone or satellite upgrades away from real-time voice communication over that.
Starlink has demonstrated a direct-to-device video call already, though, so we seem to be quickly approaching that point! My strong suspicion is that Apple has bigger plans for Globalstar than just text messaging.