If you install a letterbox you should take account of the following requirements:
Standards for letterboxes:
For full details of the regulations, consult the official texts, available via the following link: Justel database.
Operators are entitled to pursue a fair use policy for the use of roaming services within the European Union (and associated countries: Iceland, Norway and Liechtenstein).
There are two ways to implement this:
Upon exceeding the fair use policy (in any which form), the surcharges applied by the operator, may not exceed the following amounts: