Since the OOTB integration does transfer the "ca_sdm_attr_description" field value to SDM description, you only need to do as Louis suggested.
One thing you are doing wrong is : $(_.user.***) are variable that refers to the logged in user. Not the values in the textbox that the user modified!
The correct solution is :
//Get the new phone number and add a "label"
var phoneNumber = "\r New phone number : " + ca_fdGetTextFieldValue(ca_fd.formId,'txt_phone');
//Get the new emailID and add a "label"
var emailID= "\r New emailID :" + ca_fdGetTextFieldValue(ca_fd.formId,'txt_email');
//Get the text entered by the user in ca_sdm_attr_description
var actualDescription = ca_fdGetTextFieldValue(ca_fd.formId,"ca_sdm_attr_description");
//Set the text in ca_sdm_attr_description as the actual value + the phone number + the emailID
ca_setTextFieldValud(ca_fd.formId,"ca_sdm_attr_description", actualDescription + phoneNumber + emailID);
Did not test it