first of all..I envy you for having incoming calls :-)
as for the prefix- you should put one in there...in 8.10 you didn't have to, because the prefix was fixed to "9". It seems now its up to you what you want to put. Usually its a "0".
This is my best guess so far.
In 8.10 is was not necessary as you could only have one single provider line. In 10.04 you can now have multiple providers, and you can from every phone preselect what provider you want to use by dialing the prefix you specified .... So you can have 0 for one provider, 1 for another, 2 for gtalk (experimental) e.t.c.
And EU people are used to, and probably also prefer 0 instead of 9, so now everyone can do as he feels ...
Mr. Miyagi :-)