Hi Andres,
You are correct that this procedure does not work for the 7912G :(
Configuring a Custom Phone Ring
The following procedure applies to creating custom phone rings for only the Cisco IP Phone models 7940, 7960, and 7970 (and now 7941/7961).
Procedure
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Step 1 Create a PCM file for each custom ring (one ring per file). Ensure that the PCM files comply with the format guidelines that are listed in the "PCM File Requirements for Custom Ring Types" section.
Step 2 Use an ASCII editor to edit the RingList.xml file. See the "RingList.xml File Format Requirements" section for information on how to format this file, along with a sample RingList.xml file.
Step 3 Save your modifications and close the RingList.xml file.
Step 4 Place the new PCM files that you created in the C:\Program Files\Cisco\TFTPPath directory on the Cisco TFTP server for each Cisco CallManager in your cluster.
Step 5 To cache the new RingList.xml file, stop and start the TFTP service by using Cisco CallManager Serviceability or disable and re-enable the "Enable Caching of Constant and Bin Files at Startup" TFTP service parameter (located in the Advanced Service Parameters).
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a00803f3a90.html
Part of the problem is that the 7912 only supports 24 Rings whereas the other listed models support 50 Rings. You could try to edit the ringlist.xml and insert your custom rings in the first 24 (make sure you save a copy of the original)
Hope this helps!
Rob