Meeting availability using iPhone/iOS calendars
Solution 1:
An alternative would be to set up the community edition of Zarafa and only use it for calendaring. You'd either access your calendars with your iPhones or through WebAccess/WebApp (both are web-based clients for Zarafa).
Solution 2:
Check out Scheduler (http://itunes.apple.com/us/app/scheduler-by-superconnect/id546857260?ls=1&mt=8), it allows you to find common open meeting times and schedule directly through your Exchange account on iOS.