Hello,
More than 95% of the queries received concerning this issue are due to one of these two reasons:

1. The call is being attempted/made outside the coverage of the Plan.
2. The dial sequence being used is incomplete.

Either of these will result in the ''You need Skype Credit'' message being received.

To expand:
1. Most Unlimited Plans cover calls to geographic landlines only.
e.g. Only UK landlines with prefixes 01, 02 and 03 are included.
Calls to cellphones are generally not included.
Exceptions to this rule are calls made to the following six countries where all landlines and cellphones are included:
USA, Canada, Singapore, Hong Kong, China and Thailand.

2. To dial a phone number using Skype it is necessary always to include the country code prefix.
e.g. The US number 222 333 4444 should be dialed +1 222 333 4444
This applies even if you are calling locally within your own country, area code or even calling your next door neighbor!

If your issue is not covered by either of the above and you have not exceeded any of these 3 limits:
http://www.skype.com/intl/en/legal/terms/fair_usage/
please file with Customer Support here:
https://support.skype.com/support_selection