Error 603 is returned when for some reason the call was rejected by the server. Probably in your case this is due to the fact that some additional symbols persist in the numbers from your contact list like empty spaces, brackets, etc.
What you may do is to add all the additional symbols under the "Strip dial character" feature of Zoiper for iOS. This could be done in the "Number rewriting" section from the account configuration. Once you configure it properly it should strip any symbol which you have listed from the numbers and Zoiper should be able to dial.