Adding to the previous reply, the 30 day pass for $40 comes with 100 calling minutes with anything above that billed at the rate of $1.79/minute. It also comes with 100 sent texts with anything above that billed at the rate of $0.50/text. It also comes with 100 MB of data with anything above that billed at the rate of $25/100MB.
The $10/day rate uses your own plans minutes/texting/data allowances and overage rates.
While the $40/month plan may seem cheaper at first glance, overages can add up quite quickly.
For example, you say 5-10 calls/day. If any of those calls are for a duration of more than a minute, you WILL have minute overages. Using data to supply maps needed for travel, it is quite likely that you will have data overages, too. With 50(5 calls/day) calls over 10 days at just 3 minutes/call, you would have $89 in minute overages. With JUST 100 MB in data overages, you would have $25 in overages. Adding that to the $40 monthly fee would bring your international cost for those 10 days to $154.