Siemens optiPoint 500 Quick Reference Manual page 40

Before installing this equipment, users should ensure that it is
permissible to be connected to the facilities of the local
telecommunications company. The equipment must also be
installed using an acceptable method of connection. The
customer should be aware that compliance with the above
conditions may not prevent degradation of service in some
situations.
Repairs to certified equipment should be coordinated by a
representative designated by the supplier. Any repairs or
alterations made by the user to this equipment, or equipment
malfunctions, may give the telecommunications company
cause to request the user to disconnect the equipment.
Users should ensure for their own protection that the electrical
ground connections of the power utility, telephone lines and
internal metallic water pipe system, if present, are connected
together. This precaution may be particularly important in
rural areas.
Caution: Users should not attempt to make such connections
themselves, but should contact the appropriate electric
inspection authority, or electrician, as appropriate.
40 optiPoint 500 Display Phones