Where do you want to use the satellite phone? Globalstar and Thuraya might be options, but for a circumnavigation and anything in the high latitudes Iridium is the only option.
I have an Iridium with a fixed number of prepaid minutes that works out to something over $1 per minute. The minutes expire after 1 year, unless one purchases new minutes, in which case the new and old minutes get extended.
The other systems including the INMARSAT packages all require a monthly payment (at least I found no similar pre-purchase package without a contract) and that might work out more expensive for only occasional use.
I have an Iridium. I still think its the greatest thing since sliced bread when cruising out of cell phone range. Unbelievably convenient to be able to make a phone call anywhere you want. Well worth the cost to me.
We ran into a little of that when we first got it. My is a prepaid account and I guess it's around 1.50 per minute. I'm old though, and I remember when cell phones were that much a minute and I still don't talk that much on them either.
Right now, my brother has borrowed it because he is working in Alaska, and is frequently in no cell phone coverage areas.