Like any other wireless device, your reception is only as good as your connection, and with satellite phones being a direct connection device, signal loss can occur as the satellite you’re connected to moves through its orbit.
This can be combated in a number of ways. For one, ensure that you have as wide of a horizon as possible, with a clear view of the sky. Angle your antenna so that it points toward the sky, or if your phone hosts as an eCompass function like Inmarsat devices do, use it to orient yourself toward the satellite.
Iridium sattelites are constantly orbiting and are linked, so service may wane for a few seconds, but will typically recover. Inmarsat satellites, on the other hand, are geosynchrous, meaning that they are a bit tougher to connect to, but once connected, tough to loose signal.