My Iridium 9555 or 9575 handset can get a good signal off the cradle but when docked, I get intermittent signal and sometimes no signal at all.

The Iridium 9555 and 9575 handsets have electrical contacts (gold pins) that provide electrical connection to a DOCK or to an Antenna Adaptor (provided with the handsets).

As these contacts (gold pins) are exposed to the environment, they can get dirty and may prevent a good electrical connection between the handset and the DOCK. Sometimes, the more use the handset gets with those contacts, it cannot be avoided that the contacts will wear and may not offer good electrical connectivity to the DOCK.

The External Antenna Connector of the 9555 handset is at the backside of the handset, just above the battery.

The External Antenna Connector of the 9575 handset is at the bottom of the handset, just next to the gold contact pins.

Make sure the External Antenna Connector is clean and free from any foreign matter that may inhibit electrical connection. Connectivity of the External Antenna Connector can be verified with the Antenna Adaptor accessory that came with the handset.

The 9575 comes with a Bottom Adaptor that will allow you to use an external antenna while the 9555 comes with a backside Clip Adaptor for the same purpose. If using either adaptor still gives you no signal, then have your handset checked by your supplier or by an Iridium authorised service centre.