configuring linksys dsl modem router

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a customer of mine had just subscribed to adsl broadband internet with a provider called telkom. He happened to have bought a wireless linksys dsl modem router and when the telcom guys came and tried to installed it they could not and reasoned out that the modem was already locked by a different provider. The tecnician used his modem and tested that the connections was working fine with their modem. My customer was still unsure wether to buy their modem and so he rang me up for advice. So the qustion is can a provider lock your modem? I was still thinking that this telcom guy was bullshiting