E.123 specifically recommends that:
- "only spaces be used to visually separate groups of numbers "unless an agreed upon explicit symbol (e.g. hyphen) is necessary for procedural purposes" in national notation."
- "only spaces should be used to visually separate groups of numbers in international notation.
In national notation which follows E.123 recommendations, parentheses are used to indicate digits that are sometimes not dialed.
Parentheses are not allowed in the international notation, according to the standard.
A slash (/) with spaces on either side may be used to indicate alternative numbers. (i.e. "555 1234 / 4444" means 555 1234 and 555 4444.)