If your account cannot register for some reason, you may try to alter STUN and rport from Zoiper's "Settings"->"Accounts"->your account->"Network Settings".
Please also make sure that the routing device is not blocking ports used by Zoiper. The default ports used by Zoiper are:
SIP port is 5060
IAX port is 4569 UDP
RTP port is 32000 and above UDP
Default STUN vallues:
Server hostname/IP :stun.zoiper.com
Port: 3478 UDP/TCP
Refresh period: 30
Keep also in mind that some mobile operators restrict the use of VoIP into their networks. Please check with your vendor.