Whether the modification action could be filed in Ga or in Mississippi would be largely dependent on the particular facts, incl. the level of contact by the custodial parent with Ga, and where the child(ren) reside(s). Based on the foregoing, that would affect whether it is proper to require the custodial parent to appear for a court hearing. I strongly advise that you hire a family law attorney in Ga or in MS, depending on where the action is filed.
If there is a pending custody case involving a child, then the child's legal custodian generally must appear in court, except perhaps in certain circumstances where there may be a question of jurisdiction. However, based upon your question, the court in Georgia could still be the appropriate court to hear a custody action involving the children, if the parties were divorced in Georgia.
If you are involved in this custody case, I strongly advise you to speak with an experienced custody attorney in the state where the case is pending for more information.
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To answer the heart of your question, if a petition is filed and you receive a summons you must appear. Now come the exceptions that relate to the fact as to whether or not it was filed in the right place, GA or MS, you can do special appearance, the can waive presence on hardship etc.
If there has been no filing yet and you are the custodial parent in MS, here is what I would recommend:
1. Consult with an attorney in MS regarding your concerns;
2. Ask that attorney to look into jurisdiction and venue questions;
3. After meeting with and attorney make sure your are in the loop so you receive notice as soon as something is filed.
4. If filed in GA, call a GA attorney, if in MS, contact the first attorney you met.
This answer addresses only general circumstances and is not intended to be actual legal advise. Before acting on this or any other information, please consult an attorney directly.