Does switching to VoIP necessarily mean eliminating your traditional PSTN (landline) phone service? Should you dump it or keep the line? The answer cannot be a clear yes or no, although it's already is so clear-cut for many people. It is sometimes wiser to keep a basic PSTN line just in case.
There are a number of factors you need to consider before cutting down the landline cord. The only reason you will want to stop your landline service if you are using VoIP is to stop paying monthly bills on it, but the following issues will make you give a second guess.
Power OutageVoIP phones and other equipment (phone adapters, routers etc) need to be electrically powered, which makes it a big problem when power goes out. True some phones are equipped with batteries, but that cannot compare with a landline phone that has absolutely no dependency on the electricity supply in your house/office.
911 Emergency ServiceDespite claims that 911 is well supported on VoIP, it is not always true. What happened in Alberta in early 2008, where a small boy died after his supposedly-life-serving ambulance was misled by wrong 911 addressing due to VoIP, has alarmed many concerning the 911 problem over VoIP. Normally, with traditional landline, a person's address is physically linked to their phone number. With VoIP, the number is linked to an IP address instead, which is by extension linked to a geographical address, and the latter can change any time. That's what happened in the Alberta case.
Besides, many VoIP service providers chose not to offer any support for 911. That changed only when the FCC ruled to make it compulsory over all providers to support e911. Still, whether it can be completely trusted is largely debatable.