Few questions that you may or may not want to answer on here.
Who is their telephony provider? Is this a landline or mobile?
What type of business? Could they afford to shutdown their phone number for a few days if they notified their customers? Could they change the number on their website for a brief period of time(Directing calls to a new number)? All depends on where they get their leads from. Maybe setup a quick IVR to deflect the robo calls? Press 1 for sales, press 2 for support, etc. could filter out the robots?
Unfortunately, anti-spam in telecom is limited, especially if they are spoofing the number and changing them up. A service like nomorobo.com might be helpful, but again, depends on their existing infrastructure.
Happy to help the situation. I've been in telecom on the engineering side and sales side for 10+ years. Email in profile.