UK Mobile Networks
O2 prefix *0#
Orange prefix RCT
T Mobile prefix *0#
Virgin prefix *0#
Vodafone – No code exists for Vodafone
Channel Islands; Jersey Telecom *0#

US and Australia Cell Networks
US T–Mobile prefix *noti#
Australia Three *not#

European Mobile Networks
Belgium Base prefix *N#
Belgium Proximus prefix *R*
Czechoslovakia o2 prefix YYYY
Denmark prefix *0#
Denmark Bibob prefix *KV#
Denmark Three prefix *NM#
Finland Sonera prefix *0#
Germany T-Mobile *T#
Germany Vodafone D2 prefix *N#
Germany O2 prefix *N#
Germany e-plus prefix *N#
Greece Vodafone GR prefix *#
Greece Wind prefix *0#
Greece Cosmote *0#
Holland T-mobile prefix NM#
Hungary T-mobile prefix *KV# or
Ireland O2 prefix *R*
Ireland meteor prefix *R*
Italy Wind prefix *n#
Luxembourg Tango prefix *0#
Norway Telenor k#
Poland PlusGSM prefix XYZ
Portugal Vodafone prefix *NOT#
Romania Orange prefix *NOT# or *NM#
Slovakia Orange prefix *?#
Spain Movistar prefix *N#
Spain Yoigo prefix *N#
Sweden Telia prefix *KV#
Sweden Comviq prefix KV#
Sweden Telenor prefix ##
Switzerland Swisscom prefix *NOT#

Other Mobile Networks
Vodacom / Mtn prefix rrr
Singapore SINGTEL prefix #R#
Brasil Oi prefix *N#
Brasil Clario prefix *N#
Israel Orange prefix NM#
India Vodafone prefix *NOT#
Qatar Qtel prefix *N#
Maxis Malaysia *N#
Egypt vodafone prefix *VF#
Egypt Mobinil prefix *NM
India Ideal prefix YYYY
India Vodafone prefix *NOT# or *N#
Lebanon MTC Touch prefix *N#
Angola Unitel prefix *N#
Algeria Djezzy prefix *N#
Indonesia telkomsel prefix *N#
Indonesia Satelindo prefix R#
Serbia Telenor prefix *!#
Serbia MTS prefix *!#
Croatia T-Mobile prefix *r* or *r#
Croatia VIP prefix !
Turkey Turkcell prefix *N#
Bangladesh Grameenphone prefix YYYY
Bangadesh banglalink prefix *n#

can't find one for at&t USA though :P