All you need to do is go to a new carrier and sign on for service with them. As someone else stated, have a copy of a bill from your previous carrier. Your new carrier will generate a Letter of Authorization for a number port, you sign the LoA. Then the LoA and the copy of your bill that clearly shows the Billing Telephone Number and/or the number being ported are submitted by an agent or the carrier to your old carrier. If all the information on th LoA and Bill is correct, you number will be ported to the new carrier. The process takes 1-7 days in the US typically. If there is any info wrong on the LoA or bill, the old carrier has up to 30 days to notify of a failed port and you have to repeat the process with the correct info
I have 3 LoA's and 3 bills sitting on my desk, next to my work SIP phone, that are being submitted today for number ports. One is waiting for the customer to resubmit the correct bill since the number being ported does not appear on the bill they submit. The original carrier was slick and used the BTN of the customers DSL service for the number being ported.
My work SIP phone has several inbound paths, Toll free, DID, SIP, etc... It also routes calls to different places like my cell phone, other SIP Phones inside our company and even a remote extension at home. All my call forwards are setup and controlled by me. The question of whether or not I need a phone at my desk is as pointless as asking whether I need a computer at my desk. I need a device for voice communication when I'm at, near, or under my desk... and a desk phone happens to be the best device for the job. I'm the opposite with my computer, my desktop PC is relegated to basic functions, my laptop is the one I use to do all my real work.