My modem/router plus into a phone jack outlet in my wall (the only functional one in my house.) Now I need the same outlet to send occasional EKGs to a doctors office or fax. I have an adapter whereby I can plug two appliances (router and EKG monitor) into the same phone jack outlet. My question is whether doing so will degrade my wireless coverage/ communications via the router.
Yes, you can use a dual phone jack adaptor to plug your DSL modem and another telephone device into the same jack. You will need to use a DSL filter on the other device, otherwise the DSL signals can interfere with the operation of the other device and they may be degraded enough to prevent the DSL modem from working.
You should have gotten some DSL filters when you had the service installed. If you don't have one, you can find them online easily enough; or you can pay significantly more to buy one at your local electronics retailer.