Given Deutsche Telekon wanted to dispose of the T-Mobile brand in the UK it is more than likely once all investment and cost savings are complete Orange will remain the brand name, in terms of product portfolio, Orange's is considerable more diverse than that of T-Mobile. Furthermore in terms of engagement with the wide UK base Orange has the bigger foothold.
I could be wrong however, but it makes me wonder what this will do to "The Agenda" that was propagating throughout Orange, if the two companies do merge there will be some huge changes. I suspect as well however that offshoring will cease and T-Mobile's manila base will close for pay monthly.
This is all an educated guess, as the stronger of the two brands Orange will survive, France telecom will not want to kill off Orange in the UK nor anywhere else given it nearly killed the brand and has spent an awful amount of resources trying to rebuild it.
However am speculating, what worries me more is the job security. I have been employed by the company (Orange) near 10 months now and for everywhere there is an Orange shop there is a T-Mobile one close by. Which will be the first to go? I do not know.