There aren't T-Mobile specific APIs, but there are a set of APIs for the
Phone Edition devices for SMS, Phone and SIM capabilities. I wrote an entire
library for OpenNETCF to wrap these for .NETCF - see OpenNETCF.Phone within
the SDF for details.
With some limitations you can test these on the Phone Edition emulators
http://blogs.msdn.com/windowsmobile/archive/2004/04/27/121497.aspx
But at some stage you'll want to test on the same devices you'll be running
on.
Within the OpenNETCF.Phone library there currently isn't any functionality
to determine if the Phone functionality is available - if you call this
stuff on a non-Phone Edition device you'll generally get a
MissingMethodException. A quick way to determine if the device is Phone
Edition is to check if Phone.dll or sms.dll are present in the \Windows
folder - e.g. using System.IO.File.Exists
Peter