thanks for your response. Here is what I did with still the same error:
$ git checkout 3.3.2
M submodules/linphone
HEAD is now at c7186b6... Update version number
$ git submodule sync
Synchronizing submodule url for 'submodules/bcg729'
Synchronizing submodule url for 'submodules/bcmatroska2'
Synchronizing submodule url for 'submodules/bctoolbox'
Synchronizing submodule url for 'submodules/bcunit'
Synchronizing submodule url for 'submodules/belcard'
Synchronizing submodule url for 'submodules/belle-sip'
Synchronizing submodule url for 'submodules/belr'
Synchronizing submodule url for 'submodules/bzrtp'
Synchronizing submodule url for 'submodules/cmake-builder'
Synchronizing submodule url for 'submodules/externals/antlr3'
Synchronizing submodule url for 'submodules/externals/bv16-floatingpoint'
Synchronizing submodule url for 'submodules/externals/codec2'
Synchronizing submodule url for 'submodules/externals/ffmpeg'
Synchronizing submodule url for 'submodules/externals/gsm'
Synchronizing submodule url for 'submodules/externals/libjpeg-turbo'
Synchronizing submodule url for 'submodules/externals/libupnp'
Synchronizing submodule url for 'submodules/externals/libvpx'
Synchronizing submodule url for 'submodules/externals/libxml2'
Synchronizing submodule url for 'submodules/externals/mbedtls'
Synchronizing submodule url for 'submodules/externals/opencore-amr'
Synchronizing submodule url for 'submodules/externals/openh264'
Synchronizing submodule url for 'submodules/externals/opus'
Synchronizing submodule url for 'submodules/externals/speex'
Synchronizing submodule url for 'submodules/externals/srtp'
Synchronizing submodule url for 'submodules/externals/vo-amrwbenc'
Synchronizing submodule url for 'submodules/externals/x264'
Synchronizing submodule url for 'submodules/linphone'
Synchronizing submodule url for 'submodules/mediastreamer2'
Synchronizing submodule url for 'submodules/msamr'
Synchronizing submodule url for 'submodules/mscodec2'
Synchronizing submodule url for 'submodules/msopenh264'
Synchronizing submodule url for 'submodules/mssilk'
Synchronizing submodule url for 'submodules/mswebrtc'
Synchronizing submodule url for 'submodules/msx264'
Synchronizing submodule url for 'submodules/oRTP'
$ git submodule update --recursive --init
fatal: reference is not a tree: c0e20665031cdc6dadd245ebf99daf4cb4118b4f
Unable to checkout 'c0e20665031cdc6dadd245ebf99daf4cb4118b4f' in submodule path 'submodules/linphone'