If you use a lot of satphone minutes within a 12 month period, then pre-paid minutes will likely be the cheapest per-minute air time. Typically around USD $500 for 500 minutes (which expire after 12 months).
We use very modest air time, so what works best for us is a “pay as you go” plan. The best plan we have been able to find is the “Casual Plan” offered by Australia’s TR Telecom “Iridium satellite call plans Australia “[PDF].
There is NO activation/cancellation fee, but there is a four month minimum at AUD 30/month, which includes $10/month call credit. Any time after the four months minimum period, we have the option to cancel when we are done passaging, or to suspend service, for AUD 15/mon — which allows us to keep the same number. Air time for voice calls is AUD 1.98/minute.
We have hard-mounted our Iridium so it has a high gain external antenna and is always on the charger and always connected to the serial-USB hub if we need to download e.g., a GRIB and Steve isn’t willing to wait until propagation improves. If we need to make a voice e.g., med emergency call we have to sit on the port hull steps 🙂 We use Sailmail (or Winlink) first, Inmarsat-C second, Iridium third or for data too big for Sailmail. If we wish to get a quick note off to our weather router, Rick Shema, we just use Sat-C because the msg is short, and we know we will get an alert soon as Rick’s msg comes in.