The SECN code is on Github at:
For the MP02-Phone version you need the "secn_2.1" branch:
As Steve has mentioned, we are not permitted to release the SiLabs source code in this way, so that code is missing from the branch until we get an agreement in place to release it in a binary form.
If the changes you are wanting to make are not within the code that directly controls the SLIC, then the base code in the secn_2.1 branch may be of use to you to develop a patch.
If you do develop changes to the code, please consider making them available to the wider VT community in the Open Source spirit of the project.