https://sources.debian.org/patches/linphone-desktop/4.2.5-3/0001-do-not-build-linphone-sdk.patch/

Index: CMakeLists.txt
--- CMakeLists.txt.orig
+++ CMakeLists.txt
@@ -54,7 +54,7 @@ else()
 endif()
 
 
-set(CMAKE_INSTALL_PREFIX "${APPLICATION_OUTPUT_DIR}")
+#set(CMAKE_INSTALL_PREFIX "${APPLICATION_OUTPUT_DIR}")
 
 if( APPLE )
 	set(LINPHONEAPP_MACOS_ARCHS "x86_64" CACHE STRING "MacOS architectures to build: comma-separated list of values in [arm64, x86_64]")
@@ -99,11 +99,6 @@ endif ()
 #------------------------------------------------------------------------------
 # Prepare gobal CMAKE configuration specific to the current project
 
-if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
-	set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/OUTPUT" CACHE PATH "Default linphone-app installation prefix" FORCE)
-	set(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT FALSE)
-endif()
-
 if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
 	message(STATUS "Setting build type to 'RelWithDebInfo' as none was specified")
 	set(CMAKE_BUILD_TYPE "RelWithDebInfo" CACHE STRING "Choose the type of build, options are: Debug Release RelWithDebInfo" FORCE)
@@ -206,11 +201,6 @@ endif()
 if(NOT APPLE OR MONO_ARCH)
 	add_custom_target(linphone-deps)
 	if(NOT LINPHONE_QT_ONLY)
-			function(add_linphone_sdk)
-					set(CMAKE_POLICY_DEFAULT_CMP0077 NEW) # Prevent project from overriding the options we just set here
-					add_subdirectory("linphone-sdk")
-			endfunction()
-			add_linphone_sdk()
 
 			if(ENABLE_QT_KEYCHAIN)
 					function(add_linphone_keychain)
