Do these ISPs not supply broadband modems anymore? I'm seriously surprised at the amount of people running out to buy their own modems as opposed to using the ISP's generally supplied modem.
On top of that the best I've seen is a one time credit for buying your own modem and then you go back to paying for the rental service anyhow.
To top it off, the ISP generally will replace your modem as needed which says free upgrade when it comes that time.
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It's all too easy to buy the wrong thing -- the shops assume that everyone wants an ADSL unit.
And the terminology is so loose that what should be called an "ADSL modem/router with wireless" just gets called a "router".
The problem with ISP supplied stuff is that much is custom firmwared -- and can be a real pest to unlock if you want to use it with a new provider. So thousands of these effectively useless but working devices end up dumped in the garbage or sold on to the unwary.