How to automatically use SKYPE when calling international number?
On Android => 2.3 there is an option in phone settings under Internet Call Settings to add VOIP accounts as well as option Use Internet calling with the following options:


For all calls when data network is available
Only for Internet calls
Ask for each call


Using the third option is the closest you can get currently to your required way of doing this, using a regular SIP instead of Skype.

*Note: Some devices might not have these options even though they are later versions.