I had a a pack with three for un'ltd internet on my phone, but when you use the phone as a modem, you get charged and it states this in the small print too.
You get a dongle for free with contracts, the same as you get a free phone with a contract.
They are around £80 for dongle, but you need to get the one for the network, and they are all around the same price.
However, if the sim is a data sim, then maybe using a phone as a modem is no trouble and you'll not get charged per mb like i did.
I used it as a bluetooth modem too, and it was really, really slow. So use a USB Cable.