Zoiper free for linux does not appear to be respecting dns srv records. I have the following records:
$ host -t srv _sip._udp.testzones.ppbx.biz
_sip._udp.testzones.ppbx.biz has SRV record 10 1 5060 kz1.den.kc3.forethought.net.
_sip._udp.testzones.ppbx.biz has SRV record 20 1 5060 kz1.chi.kc3.forethought.net.
I turned off the registrar at kz1.den.kc3, but Zoiper never seems to try the kz1.chi.kc3 address.