One unfortunate aspect of every system administrator's life is the adage, Murphy's law:
"Anything that can go wrong, will go wrong."
Therefore with that in mind, it's essential to be prepared to understand the basic troubleshooting steps to help determine where to begin investigations when invariably an end user reports issues connecting to Exchange ActiveSync.
Mobile Devices are complex to troubleshoot as there are a lot of interdependencies that are essential for them to connect to and synchronize with Exchange Server 2010; these include:
The iOS mobile device operating system
Proximity to and functionality of local cell towers
GPRS or 3G connectivity to the cell provider network
The cell provider network's Internet connectivity
Central Internet exchange points where Internet service providers peer or connect to transit
The ISP providing Internet connectivity for the organization hosting the Exchange Server
External DNS services providing name...