Cisco Wireless :: 1242 - Max Number Of Voice Clients On AP

I have a high school wireless environment that was installed approximately 5 years ago with 1242AG APs. The install was done based on 2.4GHz coverage everywhere, not for 5GHz or for good density. The high school uses wireless cisco 7921G phones for all the staff and teachers. The issue we're running into is over the last couple years more and more students are bringing in iPhones and android phones that can connect to the public wireless ssid. And with the increase of these wireless devices in the classroom, we've been experiencing more and more call dropped on the wireless phones. I have collected information on number of clients on specific APs that are in the area of most of the dropped calls. At times some of these APs have a total of 40 clients associated to them, over half of which are on the public ssid. I have thought about putting all the phones on to the 5GHz radios, but the implementation of the wireless was only for 2.4GHz coverage, so we have coverage gaps on the 5GHz network. We're thinking about adding more APs but the budget isn't available yet for that, so we're looking at what we can do to make this better for the time being.
 
Main questions are:
 
Is there a best practices number of max clients per AP in a wireless voice network?
 
Are there any other options for preventing dropped calls on the wireless?
 
I should mention I have already looked over the VoWLAN Troubleshooting Checklist at the link below and made the necessary changes to assure we are following the items on the checklist as close as possible.URL