Trans World Systems. Calls several times a week. Always leaves the same generic message that they are calling trying to collect a debt. They can call under any of these numbers:

866.472.7870
443.552.5819 (Maryland)
443.552.5884 (Maryland)
443.552.5895 (Maryland)

According to the website, they are a medical debt collection agency, but they never leave any information about what medical provider they are calling about.

https://www.tsico.com/service-areas/collections/