In Mississippi the laws are that a landlord/management needs to provide the tenant with a 3-day notice for violations of the lease. For example, if a tenant owes rent, they have 3 days to correct the situation.
If the issue is not remedied in this time period, the next step is for the landlord/management to file for an actual eviction through the local court system. Once this filing is done, the tenant will receive a notice from the court that they've been filed against. They have a certain number of days (these will vary from state to state) to respond to the court about this notice. If they fail to do this the judgement for eviction will go to the landlord. If they answer the court, a hearing will be set up and both sides can argue their case on whether or not the eviction should occur.
Until this judgement has been granted, the landlord CANNOT change locks, toss out the tenants personal property, etc.. If they do this this is considered to be an "illegal" eviction.
Because it costs to file, I suspect that there are some landlords who skip this step and do what your landlord seems to have done; simply gone ahead and changed the locks, hoping that the tenant will not realize this is illegal without a court order that grants the eviction.
You indicated that you received a phone call (not a letter) on December 12 regarding the matter of owed rent. If I were you, I'd do nothing until I received something in writing regarding this matter.
IF what you receive comes directly from the court, it will list when this hearing will take place and you need to be present then to make certain the judge hears that a legal eviction never took place.
As a part-time landlord, I don't like to see tenants skipping out on paying owed rent but I also don't like to hear of landlords doing illegal evictions (and I suspect these occur all too frequently).
Gail