parsing letters in web service

I have a very simple web service. When i use stored procedure without serbian letters (latin or cyrilic) i get some request, with the full envelope, without errors. But, when I invoke same servise with those letters like arguments, i get error:

 <faultcode>env:Client</faultcode>
  <faultstring>Internal Error</faultstring>
</env:Fault>
l_return =

I contacted providers so they told me acctally that i have problem on my own side...

How to fix this isue?