How I get the string on faultstring

Hi guys,

I need get the string of the faultstring. How can I get it??

Example:

<faultstring xml:lang="en">The following parameters are mandatory: INSTANCE_ID, EFFECTIVE_DATE, REQUEST_ID_PUB</faultstring>

I want to know how I can obtain this string "The following parameters are mandatory: INSTANCE_ID, EFFECTIVE_DATE, REQUEST_ID_PUB"

Thanks to your help!!