I can see the REGISTER requests in my freeswitch host from Zoiper, but they never include the authenication details, so registration always fails with Unauthorized.
REGISTER sip:192.168.1.5:5060;transport=UDP SIP/2.0
Via: SIP/2.0/UDP 192.168.1.254:57309;branch=z9hG4bK-524287-1---98aed91e7a214568
Max-Forwards: 70
Contact: <sip:pete@192.168.1.254:57309;rinstance=2d703db1f60f365a;transport=UDP>
To: <sip:pete@192.168.1.5:5060;transport=UDP>
From: <sip:pete@192.168.1.5:5060;transport=UDP>;tag=7090d220
Call-ID: y0QLMkHfiaPvjFmsDabwkg..
CSeq: 62 REGISTER
Expires: 60
Allow: INVITE, ACK, CANCEL, BYE, NOTIFY, REFER, MESSAGE, OPTIONS, INFO, SUBSCRIBE
User-Agent: Zoiper rv2.8.105
Allow-Events: presence, kpml, talk
Content-Length: 0