Here’s an interesting Open Source-ish app for Pocket PCs based on Windows Mobile 5 and .NET Compact Framework 2.0. The project description reads: SMS Notifier watches for incoming calls that are missed (i.e. not answered). Depending on configuration settings it does the following things: 1) Send an SMS message to the caller (configurable contents), possibly containing also the end time of current appointment (configurable). 2) Adds an item to calendar (containing the caller info).
You’ll find a CAB file installer at the page linked above (Microsoft’s CodePlex site).