According to what you're letting us know, it's possible that you have more than one Spotify account associated to you. If you'd like, you can find some useful information on how to locate additional Spotify accounts on this page.
Once you've located the other account associated to your email address, our best suggestion would be to close it following the steps you'll find here. You'll be able to make the changes on your account as soon as that's been taken care of.
I had the same problem, I couldn't change my area code, because I had some accounts that were linked to me but I had just one phone number then. Maybe, I market it when I made a registration, but it also said that the E-Mail address is already in use. Indeed, I registered for many times my mobile contact and area code in the US on the internet for making new accounts and profile pages. So, when I changed the operator there appeared a whole problem to get access to all my pages, especially for those old because they send control codes directly in a message on your mob. umber.