It would appear that Orange does have UMA support (to makes calls via wi-fi)
and when connected to your wi-fi may show UMA in the corner.
Apparently there is some confusion over whether Orange says the 8900 is supported, but other users have got this to work.
http://www.blackberryforums.com/gene...ma-8900-a.html
http://www.blackberryforums.com/gene...my-8900-a.html