I had the same problem with the Garminfone I bought used off eBay several weeks ago. My solution was low-tech, but it worked; insert a thin item between the SIM card and the inner surface of the thin plastic SIM card holder (I used a doubled-up strip of tape). I surmised that the metal contacts of the SIM were not making firm and solid contact with the Garminfone's SIM card contacts. It worked! The only thing is, sometimes, the pressure slacks off; if that happens, just power off the Garminfone, pop off the back cover, take out the battery, slide out the SIM card, and readjust/reposition the thin item, then replace both thin item and SIM card, replace the battery, snap closed the back cover, and try powering the Garminfone on again. Usually, that solves the problem.
Let us know if that works! Good luck, and enjoy your Garminfone!
