The recent 2.07.401.1 ROM upgrade for the HTC Touch Pro 2 added a wide range of new features, such as HTC Sense 2.5. Unfortunately it also added a number of bugs. One of these is that the when the speakerphone is active the other callers may not be able to hear you - the microphone is effectively muted.
There is an easy workaround for this problem. On your Touch Pro 2 go to the home screen. Slide to the "Settings" tab. Tap the "Menu" softkey and choose "All Settings". Tap "Personal" then "Phone". Open the "Advanced" tab and change "Reduce noise in phone calls" to "Never".