can someone spying the skype calling?

Skype traffic is easy enough to inercept and monitor but this does not cause calls to be on hold.  It is pretty much invisibale to the user.
If calls are going on hold then it is a software / network issue.
But... you can be monitored very easily so do not assume skype is secure.

is there anyway to secure the traffic?

and how can I know exactly if there is someone watching the traffic or spying on me?

The traffic uses a simple form of security.  To most people it can not be spied on.  It is secure unless someone is spending time and effort or knows what they are doing.
The system they use was cracked about a year ago and if some really wants to they can listen in on you.
There is no way of knowing they have done this.  

Again really no way to tell unless you contact Skype asking if there has been a breach

Skype would and could know if a line was snoop on. The old telephone days were harder to know if a line was snoop on or hacked.

No.  They dont know.  
If you send your data from your pc to skype and I monitor the data on route - I capture the data as it passes... but it still passes intact to skype then how would they have any idea what had happened.

I do know what I am talking about here.  Skype is hackable (but it is not very easy and is not very common).  If it is monitored then no one (out of the two calling parties and skype) know that it has been done.

Again I do know what I'm talking about as well. I have a degree in Security and mutliple certifications in the field of security. I worked in Security Department and blah blah blah.

Now who knows if Skype monitoring the encryption lines but why would you have security and not monitor the lines. Either way it is a good idea to file a compliant with Skype to ask them to look over.