What's wrong of the contact string? 639(snom) works but 637(zoiper) doesn't."user" <sip:637@192.168.1.27:5070;rinstance=e1e47e9a22f3e450;transport=UDP;fs_nat=yes;fs_path=sip%3A637%40192.168.1.27%3A5070%3Brinstance%3De1e47e9a22f3e450%3Btransport%3DUDP>"seven" <sip:639@192.168.1.21:2051;line=298293g2;fs_nat=yes;fs_path=sip%3A639%40192.168.1.21%3A2051%3Bline%3D298293g2>