In addition to the guide provided by Sameh_Sackla, I would not recommend by any means using 802.11r with this Cisco Phone.
It comes from a time where 802.11r did not exist, and while it can still theoretically connect to a network where 802.11r is enabled (from what I can see in the Cisco forums) it is strongly recommended not to turn it on.
Have you done an active Site Survey? It is very important to see these parameters are on comply with VoIP requirements (plus any additional ones you see on the deployment guide, but these are the main ones):
- Co-Channel Interference (no more than three APs heard at the same time, with a difference of at least 20-25 dBm SNR on the non-overlapping part)
- Signal received (between -67 and -65 dBm over all the cell)
- SNR (more than 20 dBm if possible).
- Jitter(the closest to 0 the better). This can have a high impact on your calls.
Additionally, on the LAN side, everything should be able to handle QoS marking and prioritize the traffic accordingly to this tagging.
Here are some documentation you would like to see about how to deploy a wireless network and how to tune it:
And we also have a general guide to deploy voice over Wireless, you may find it very useful:
And two very specific - the first one is part of the deployment guide, the second one a Cisco guide for deploying this phone with Cisco Meraki wireless infraestructure.
Hope this helps