Hi Joseph,
The spa525g2 has a feature called electronic hook switch or EHS. That might be what you are looking for. this feature is setup on the voice--> user tab. This does require a headset and a special cable adapter APC45 from the headset to the phone.
Electronic Hookswitch Control Some headsets enable you to answer and end phone calls by using
controls located on the headset. Set this parameter to yes if the user
is using a headset with a control function. This feature has been
tested with Plantronics base/ headsets (that require adapter APC45):
• Savi W7xx series (Savi W710, W720, W730, W740, and
W745)
• CS5xx series (CS 540, CS 510, and CS520)
The default setting is no.
Note Applies to the Cisco SPA525G or Cisco
from the admin guide http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/csbpipp/ip_phones/administration/guide/frame_source/500-series-7-5-7/ip_phone_admin.html
hope this helps,
Dan