We can change it BUT the problem is that links (both internal on your site and on other sites round the Internet) will be using the old address, and these links will no longer work. It'll be a signnificant pain for you to change all the links in your site, and you'll lose traffic as search engines will send people to the old URL for a period until they notice the change.
I very much recommend that the best thing to do is to register your own domain name - chelfordtogether.org or whatever - and associate that with your Voice website.
This has the advantage that anyone using the old URL will still get to the site (and will be automatically redirected to your new domain), and search engine links won't get broken.
Voice will automatically register a SSL certificate for your site once you add it, so it will still be fully secure and use https.
There are lots of domain registrars around. Registering a domain is cheap and reasonably easy. I use 123-reg.co.uk for most of our domains.
Joe - Voice Admin