For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:FreeCalypso/openSUSE_Tumbleweed/home:mnhauke:FreeCalypso.repo
zypper refresh
zypper install gsm-codec-lib
For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:FreeCalypso/16.0/home:mnhauke:FreeCalypso.repo
zypper refresh
zypper install gsm-codec-lib
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:FreeCalypso/15.6/home:mnhauke:FreeCalypso.repo
zypper refresh
zypper install gsm-codec-lib