It sounds like your DSL modem was provisioned with 2 addresses from the start -- the Mac and the PC. Typically, a DSL modem will only assign one IP address. Depending on your deal with Telus, you may have been paying extra all this time for 2 IP addresses even if the PC wasn't being used.
For a device to operate on an TCP/IP network, it has to have an IP address, simple as that.
DHCP means Dynamic IP address allocation: It's an on-demand thing.
When your computer powers on, it asks the DSL modem "What IP address can I have?" and the modem says back: "Here, have 123.234.123 (or whatever number is available in its pool of numbers)" and your machine says "Okey Dokey" and loads that number as well as some other supporting settings, into the Network Preference Pane.
So there isn't a permanent IP address assigned to your Mac, or your VOIP adaptor, but each one gets a number assigned to it automatically when it asks.
A router will do the same with the DSL modem -- it will ask for a single IP address, and receive the number 123.234.123 from the modem.
But then, it acts as a middle man -- your computer(s) and VOIP modem ask the ROUTER for IP addresses, and the router creates and hands out a series of its own internal network addresses "192.168.1.101, -102, -103 etc"
-- this can be to as many machines/devices as you want. Telus still is only providing 1 IP address from their modem, so you only need to pay for one address only.
In addition, because your computers are insulated from the wild Internet by the router, it provides an extra measure of security.
Short answer - take back the switch, get your $35 back, and spend $45 on a router that includes a 4 port switch. The Telus rep (and possibly the salesperson who sold the switch) are telling you wrong.