Hi Nicole,
Sadly, the weak link here is the 7931G as it does not support
HHC - Headset Hookswitch Control or also called EHS -
Electronic Hookswitch Control which is available on other newer Cisco
models like the 7942/62, 7945/65 89xx, 99xx which allows
for exactly what you need.
In your current situation you will require the "mechanical" lifter
kit as shown with the nice Plantronics CS540;
http://headsetplus.com/product1382/product_info.html
If you have a new IP phone model you can leverage this
with the use of a special "APC" cord instead of the more clumsy
lifter
I have attached a matrix of Cisco phones so you can see which ones support HHC
Cheers!
Rob
"Hours are like diamonds, don't let them waste
Time waits for no one, no favors has he"
- Stones