A Delclined error is very vague, it just means the call was rejected for some reason.
Keep in mind that the service provider can choose to generate whatever error code they want, so 'declined' can mean something different from provider to provider, so it is up to us to try and guess what the problem is.
I'm thinking it could mean:
- Insufficient funds / account balance negative
- Username/password combination is incorrect
- You are not authorized to dial that kind of number
- The dialed number is not formatted correctly
- The callee decided to screen and reject your call
- General call failure, perhaps service-wide