Go pick up a used ATTWS tdma phone (I recommend a v60t) off ebay and go to www.ecallplus.com
They essentially re-sell ATTWS pre-paid. Their "cards" are good for 90 days, but they make reference to the fact that 'many customers find their phone works for 6 months or more.' (more on this later...)
The minutes are good nationwide, its just that outside the ATTWS network, they might have to enter a 'PIN' (which you can set) to complete the call. Voicemail is included, and you can check it for free (or at least won't burn airtime) from a landline. You do need to make *one* outgoing call every 30 days to keep the account active (you may need to burn at least 1 minute for the billing system to increment the "miuntes remaining" counter and this is probably what keeps the account active).
I did this for my parents last winter when they went to FL for 3 months (so they wouldnt have to re-sign a verizon contract in the process of changing to a nationwide plan). I re-charged it twice for them when they were down there (and if you have remaining minutes, *all* minutes buttume the expiration of the new card). The last "re-charge" was in early march. We checked it last week and all the remaining minutes are still there and their buttigned number is still there!! (If you let your minutes expire, you will often lose the buttigned # and have to get a new one when you do buy more minutes). That would be 9 months and counting on that last "card." (I buy it on the website and if you tell them your cell phone # in the comments on the web form, they just add it to your account for you within 12 to 24 hours).
They have a "starter" pack for $25 that includes activation and 100 minutes.
Verizon (and their pre-pay "partners" like TracFone) has better coverage in some markets than ATTWS, but ATTWS has extensive roaming partners in almost all markets (usually the best "roaming" networks are either ATTWS or Verizon - they have the mose extensive roaming agreements nationwide). They can't really go wrong with either - but ecallplus is probably the best deal out there right now.
This site has some info about ecallplus and a similar company - Pharos international (which sells a $10 card for the same service):
Note: Among other things, they say you *will* lose your minutes after 90 days, but this has not yet proven to be true for me....
Good luck!
Jon