Scrolling to top while on a call?
No - the functionality replaces the tap for scrolling with a tap to resume the call.

You might instead need to use spotlight to search for the contact rather than scrolling in the case you mention. Worst case is you can use the paging / "scrubbing" gesture to tap and then scroll vertically along the side of the contact application to rapidly scroll to the desired letter. For apps without this added functionalities, you must scroll or end the call and then obtain the information you need before resuming the conversation.