Obtaining a social security certificate
In the case of regulated professions and activities, the provision of cross-border services also involves a statutory obligation to inform the Social Insurance Institution about the provision of a cross-border service before commencement of the service and in the case of an emergency - after the service has been completed (pursuant to Article 31 sec. 2 pt 4 of the Act of 22 December 2015 on Principles of Recognizing Professional Qualifications Acquired in the Member States of the European Union).
In practice, this operation requires a service provider to obtain a social security certificate (A1 form) from the insurance institution in his country, competent for his place of residence. The model A1 form is identical throughout the European Union; on its basis, the service provided pays contributions in a given EU country and obtains an exemption from payment of these contributions in another country. The obtained certificate must be presented, if necessary, to an employee of the Social Insurance Institution, e.g. in the course of an inspection.
Service providers who render services in the EU internal market are obliged to pay contributions to social insurance in the member state where they conduct their "substantial business activity." Therefore, a service provider is not required to register his activity in the Polish social insurance system.
More information is included in a ZUS guide (in particular, pages 14-15 of the guide).