Getting Asterisk packages and pubkey 75307751175E41DF

If you just added the  Asterisk packages using,

sudo add-apt-repository "deb http://packages.asterisk.org/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 http://packages.asterisk.org/keys/175E41DF.pub -O - | sudo apt-key add -

You can now reload the packages within Synaptic.

See https://wiki.asterisk.org/wiki/display/AST/Asterisk+Packages#AsteriskPackages-APT%28Debian%2FUbuntu%29