There is no longer a direct download link for the Zoiper tarball.
Get it manually from:
  https://www.zoiper.com/en/voip-softphone/download/zoiper5/for/linux
and put
  zoiper5_5.3.5_x86_64.tar.xz
in your ~/Downloads folder or in with the PKGBUILD.

Or use the supplied zoipdl.sh
==> ERROR: Failure while downloading manual://zoiper5_5.3.5_x86_64.tar.xz
    Aborting...
:: Unable to build zoiper - makepkg exited with code: 1