The first mobile operator appeared in Belgium in 1994: Belgacom Mobile, which was a subsidiary of the state-owned operator Belgacom. Even if Belgacom had previously used another kind of mobiles, called Mob 1 & Mob 2, its subsidiary started to sell in 1994 what is usually known in Belgium as GSM.

Up to know, three mobile companies are operating in Belgium: Proximus (formerly Belgacom Mobile), Mobistar & Base (ex KPN Orange).
 
 

Operator	Foundation	GSM type	Postpaid	Prepaid
Proximus	1994	900 Megahertz	Yes	Pay&Go
		1800 Megahertz (in major cities)
Mobistar	1995	900 Megahertz	Yes	Tempo
		1800 Megahertz (in major cities)
Base (ex KPN Orange)	1998	1800 Megahertz	Yes	CashBase (ex PrePay)

NB: When KPN Orange launched its services, it was a joint-venture of KPN Telecom (The Netherlands) and Orange (UK). However, as France Telecom bought Orange (UK), Orange sold back its shares in the venture to KPN.
Therefore, KPN Orange Belgium was using the brand name "Orange" to sell its products... at least until 1 October 2002 (deadline to perform a brand name modification).
At the end of April, KPN Orange Belgium became Base, 100% subsidiary of KPN Mobile (The Netherlands).