Options are limited. Cisco's current answer would be Unified Mobility Advantage (CUMA). The general idea is it installs on the Windows Mobile (supported only) handset and gives you access to various Cisco UC features including soft keys.
Be advised that this involves an MCS7845, ASA, and software licensing so it is not a trivial expense.
http://www.cisco.com/en/US/products/ps7270/tsd_products_support_series_home.html
Your only other option would be to investigate a third-party SIP client that could run on the phone and register through 802.11 to UCM.