I am not familiar with the backend VoIP server and you haven’t mentioned what Apple / Droid apps you tested. You have enabled QoS on the phone apps and you have checked for the correct QoS end-to-end (IAP to switch to VoIP server)?
I've tested and seen some android softphones NOT enable QoS or fail to enable the correct VoIP QoS markings which could impact the packet priority settings and handling in the Aruba ALG. Quick packet capture over the air will show this.
I found it good practice to minimize the "IP distance" between the WLAN VoIP client and the VoIP server. If you can provide IP addresses directly from the VoIP server subnet as the VoIP client registers when connected to the VoIP WLAN.
Since it is a separate WLAN – SSID although not as secure WPA2-AES will provide a faster authentication when roaming. Unfortunately I did notice that WPA2-Enterprise did slow down the Droid CPU in early versions of the OS.
When you have reviewed the packet capture you can open the advanced option in the IAP VoIP configuration to enable and set your WMM preferences.
Of course all of this may not alleviate the roaming drops as it is the client that determines when to roam.
I have used the Bria softphone (CounterPath SIP softphone) on my android with good results for local testing in my lab to my SIP server (I have the Apple version on an iPad as well) A web search may provide other Apple / Android dedicated VoIP apps you can download and test for a limited time (trial) to find the best for your environment and devices that the user base finds acceptable.
Good Hunting