A landlord may call (or otherwise contact) the tenant's employer at any time, whenever he or she likes--or could contact the tenant's family, friends, a restaurant which the landlord knows the tenant frequents, a school the tenant attends, etc. There is no law against person A calling persons B, C, D, etc. in order to locate person E. The only real restriction in this area is if the landlord is trying to collect a debt (e.g. back rent) and is not doing it him- or herself, but rather has engaged a third-party debt collector (e.g. a collection agency) to collect for him. The Fair Debt Collections Practices Act  (FDCPA) puts limits on a third-party debt collector's ability to contact people to collect a debt. But if the landlord is trying to collect his or her own debt, these restrictions would not apply.