In that case it would help a great deal to identify the equipment that he wants to change as well as the type of internet service currently in use. For example, DSL requires an active phone line, so the phone bill would only go up. Whether or not his equipment is currently internet-ready or if it requires an upgrade would determine whether or not it's an easy change as well as the cost.
1st, I think we wants to go to voip to clear his CCs. You do not need a active phone line (wet loop) to have DSL. I currently have DSL (dri loop), so no phone line circuit attached...the DSL modems connects to the DSLAM directly.
2nd, it is possible to do this as the modems used to clear creditcards use low speed robust modems. However, in some cases, depending on internet traffic, the call could fail.
Lastly, if you want an internet connection directly from your WSII (ethernet), you can do it two ways. Get an IP modem from Unitec and connect it to your dial up modem (it has your CC clearing info). Or, you can get a washpay server and a NIC card for the WSII. This is a better option as it gives you more flexibility, reports, etc and allows you to hook up other terminals like SS bay CC equipment
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