Getting Asterisk packages and pubkey 75307751175E41DF

If you just added the  Asterisk packages using,

sudo add-apt-repository "deb `lsb_release -cs` main"

and are now trying to install Asterisk from packages and you get an unsigned key problem,

NO_PUBKEY 75307751175E41DF

then you must do as root,

 wget -O - | sudo apt-key add -

You can now reload the packages within Synaptic.