Depending on the term "back-up" that is being used here, the method of backing up calls or databases would vary depending on the customer. In this case, I would say not enough information if provided to give a detailed suggestions. But the overall answer to the questions is Yes, you can backup the calls, the databases or anything else on your servers. Exactly how that would done would require more information. Personally, and logically, I think a general server back plan is sufficient, ensure my databases are getting backed up and of course your calls should be on an appropriate RAID setup for critical data. And I would not wait nor expect any vendor to tell me how to perform my backups. My 2 cents.
-Plausible deniability is not very becoming...