For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:telephony/openSUSE_Tumbleweed/network:telephony.repo
zypper refresh
zypper install sipp
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:telephony/15.6/network:telephony.repo
zypper refresh
zypper install sipp
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/network:telephony/15.5/network:telephony.repo
zypper refresh
zypper install sipp