We have a Mitel 5000 phone system (MiVoice Office 250) with Ascom i62 handsets on ZF R500 network of AP's. The i62 handset signal is poor compared to smartphones with SIP client, and frequently drops signal
This isn't surprising at all. The received signal on the devices is the same, but client devices have different radio designs and their actual receive signal can vary even up to 10dB in signal strength.
What you NEED is to understand that, and then design your WLAN network accordingly to the clients that will be deployed in your environment. If you haven't done so, get a true WLAN professional. Sure they cost money but still way less than an amateur.