[my_pointy_head_is_sharp]

Then why wouldn't we have bugged Dodi's phone, and not Diana's?

[A knight without armor]

Because Diana may have been talking to him on her phone or talking about him on her phone. Plus the article doesn't say the U.S. wasn't bugging Dodi's phone, too. At least I did not catch that. Correct me if it does.

[RightOnTheLeftCoast]

"Then why wouldn't we have bugged Dodi's phone, and not Diana's?"

Let's pretend for the moment that Dodi Fayed's name had come up in the investigation of funding of, oh say Ramzi Yousef, the WTC bombing mastermind, or something like that. (Warning: this is entirely conjectural for illustrative purposes only. I am making no accusations.)

Certainly, in such a case those around him would be subject to surveillance also: If you're a bad guy and want to make a phone call to another bad guy, it would simply be good craft to borrow an innocent's phone. And it would simply be good practice to monitor the phones and activities of potential dupes in frequent contact with your suspect.

Again, I'm not saying Fayed was any sort of bad guy. I'm saying what-if, and explaining how bugging Di's phone makes eminent sense if Fayed was any sort of suspect. Which, as an staggeringly wealthy muslim, he may well have been, if he'd given money to the wrong people, even naively.