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.
