Error SIP 603 is usually returned as a response when the called party was successfully contacted, but can not or does not wish to participate. This error message is sent by your VoIP server and Zoiper is just displaying it.
This usually means that the user has rejected your call, but could also mean that the provider rejected the call due to insufficient balance in your account or other reasons.
Please contact your VoIP service provider or PBX administrator for more detailed information regarding this error message.