Mike,
Your original question was can I change CALLSIGN / DVSW to something else. Well, No, your callsign is a required field in the protocol and most networks verify that it is valid and allowed. The /DVSW field is optional, but is limited to 4 characters and most DSTAR users set this to the radio that they are using (ID51, etc). Since you are using DVSwitch Server, I set this to conform to the norm.
Now, you say that you are getting what looks like garbage and NOT callsign/DVSW? I have not seen that and would like to see your MB logs surrounding a short transmission.
Mike