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 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
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:FreeCalypso/15.5/home:mnhauke:FreeCalypso.repo
zypper refresh
zypper install gsm-codec-lib
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:mnhauke:FreeCalypso/15.4/home:mnhauke:FreeCalypso.repo
zypper refresh
zypper install gsm-codec-lib