I ran into this exact issue, in Canada, with an old fashioned POTS line and DSL internet, but I suspect the same root cause. I suspect the number cannot be ported while it has another service on it... DSL. What I did, I switched to cable internet first which allowed for a couple weeks overlap on the DSL as well so I would not have an outage. About a month or so after the DSL had been cancelled I checked again and magically my number was now able to be ported, so I then switched to the VoIP service of my choice. It just took an extra step.