Without knowing the nature of the business - manufacturing, IT, , support, customer service, etc. - my first inclination is to suggest that you operate on the same level as your customers, or industry. After all if there is no one there when the phone rings, the caller will go on to the next number, and your firm has lost a potential opportunity.
Having said that, you may want to consider a "rotating weekend", where a percentage of the workforce (25%) has the "duty" while the rest are off for 2 days. Under this plan, an employee would have the benefit of 3 "short" weeks a month, while the company still has someone to "answer the phone".
With the "short" weeks, they can plan to spend more time with the family, travel, meditate, and in general relax and rejuvenate from stresses at work as well as prepare for the week ahead
The employer gets the benefit of a well-rested employee, freed from stress and thereby physical and mentally prepared for the work ahead.
Another thought: Why Saturday-Sunday, and not Sunday-Monday? The latter would allow employees to take care of personal business on their own time, potentially reducing absenteeism during the regular workweek.
Of course this schedule would dictate the level of "on duty" personnel to raise to 50%, and the rotation would be weekly or bi-weekly, according to the input from employees, and the needs of the business.
Hope this helps.
PALADIN