Since iOS 10, iOS devices have the possibility to show a small webpage preview in the Messages App, this is usually not helpful for SightCall links, as there is nothing to show for that preview. In order to make sure that this doesn't happen, you just need to add some text before and after the URL.
This means that the SMS notification will have to look like this:
Dear %DISPLAYNAME%,
click on the link to activate your account on SightCall:
%REGISTRATION_LINK%
Thanks,