If you click the buttons on your uCall website after choosing a sub-domain, you might see the site domain changes to something like u820645.umbrella-sites.com instead of showing a url containing your chosen sub-domain.
To fix, simply double click the button(s), and in the edit popup, scroll to anchor link.
Once there change the part of the link that reads uXXXXXX (the letter 'u' and 6/7 digits following) to your subdomain.
E.g. instead of u820645.umbrella-sites.com, replace u820645 with your sub-domain name > mysubdomain.umbrella-sites.com
Click Save on the edit popup and click Publish in the top right navigation bar.