I have connected a cisco Linksys ethernet switch to my comcast cable modem to link computer and Cisco IP phone, along with another computer also. The Cisco switch had 5 ethernet ports. I've tried every combination with the phone, switch and internet using instructions from all sources and no matter what I try, or what combination I use to hook up switch with phone and internet, eather only the phone works, or only the internet works. I can not get the two to work at the same time through the Cisco Linksys SE1500 switch. Each works independently when connected directly to the modem. It's impossible to get help from Comcast/Xfinity - I get no response and I'm afraid that if I get someone on the phone from the ISP, I wont be able to understand a word they say since 99% percent of them dont speak proper english. Any advice?? I'm about to look into a wireless system to work with my ISP. Is it worth it or will I run into the same issue?
I have a feeling the reason you are running into the problem is because you are using an ethernet switch but your ISP only supplies you with a single IP address and therefore only one port of the switch will provide a connection. If your ISP provided 3 IP addresses, 3 of the ports on the switch would have a connection. Most ISP's only provide a single IP address per subscription and people use routers with DHCP and NAT (network address translation) to connect multiple devices.
When a router like this is used the router takes on the public IP address provided by your ISP and then issues private IP addresses to each of the connected devices and routs traffic to and from the appropriate device and the internet. The outside world only sees one device - the router but behind the router is your home network. This is actually a secure and convenient way of accomplishing the goal of connecting multiple devices since each of your computers/devices is not directly connected to the internet and the actual details of your home network are not exposed directly to the internet.
If the above situation actually corresponds to your situation buying a wired or wireless router will likely solve the issue.
If on the other hand the device you are using is a "router" and not a switch and you are having this issue it is likely the case that your CISCO IP phone requires some special port forwarding since it is not directly exposed to the internet and requires some special setup in order for the router to properly route the required data.
Are you using an ethernet switch or an ethernet router?