In order to configure your account in Zoiper, go to "Settings"->"Accounts" and select Add account.
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Select No then
"Manual configuration".
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After selecting "Manual Configuration" and choosing the account type (SIP) you need to fill in the following fields:
Account name - choose a name for your account. (you can use any name)
Host - sip.tkhost.com
Username - your VoIP account username.
Password - your VoIP account password.
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add the same username to "Authentication username"