Lets get list of packages installed by asterisk

root@cloud1:/var/log/apache2# dpkg -l | grep asterisk
ii  asterisk                                  1:1.8.13.1~dfsg-1ubuntu2                  amd64        Open Source Private Branch Exchange (PBX)
ii  asterisk-config                           1:1.8.13.1~dfsg-1ubuntu2                  all          Configuration files for Asterisk
ii  asterisk-core-sounds-en                   1.4.22-1                                  all          asterisk PBX sound files - US English
ii  asterisk-core-sounds-en-gsm               1.4.22-1                                  all          asterisk PBX sound files - en-us/gsm
ii  asterisk-modules                          1:1.8.13.1~dfsg-1ubuntu2                  amd64        loadable modules for the Asterisk PBX
ii  asterisk-moh-opsound-gsm                  2.03-1                                    all          asterisk extra sound files - English/gsm
ii  asterisk-voicemail                        1:1.8.13.1~dfsg-1ubuntu2                  amd64        simple voicemail support for the Asterisk PBX
root@cloud1:/var/log/apache2#

To uninstall a package, you need to use

apt-get remove PACKAGE-NAME

So, lets first remove asterisk package.

apt-get remove asterisk

In most cause, it will remove all dependency. If not, check with dpkg -l and un install remaining packages.