HTC-Desire-HD Calls go to answer phone if emails / data is updating a customer of mine has a HTC-Desire-HD that when Data is updating be it emails, weather or any other data app the customer can not receive any voice calls as they go straight to answerphone..

Data turned off the calls work no problem.  

If the phone is ont he home screen or other menu screen the phone calls are fine.

only when data is in use does the call go to answerphone.

Any tips??
Only dual-transfer mode phones and networks can use both voice and data at the same time.  I can't find any hard details on the Desire HD but I assume it is not DTM enabled/capable.