UC500.com

I don't believe the 7961G supports the 7915.

Reference:
The Cisco Unified IP Phone Expansion Module 7915 extends the capabilities of the Cisco Unified IP Phone 7962G, 7965G, and 7975G models with additional buttons and an LCD display. With this expansion module, you add 12 physical keys with access to 12 additional keys through the page keys for a total of 24 buttons to the existing six buttons of the Cisco Unified IP Phone 7962G (Figure 2) and 7965G models and the existing 8 buttons of the Cisco Unified IP Phone 7975G. You can use up to two Cisco Unified IP Phone Expansion Module 7915 modules (Figure 3) with the Cisco Unified IP Phone 7962G, 7965G, and 7975G models.

http://www.cisco.com/en/US/prod/collateral/voicesw/ps6788/phones/ps379/d...

We have no issues running the 7915 on 7962's with the UC if you upgrade the Firmware manually to 12.4(20)T and drop in the relevent, updated phone loads and 7915-12/24 load.