An Overspell is a phone number that uses extra phantom digits to complete the spelling of a word. For example 1-800-ESURANCE (1-800-378-7262), the word ESURANCE is a vanity number with 8 characters (1 extra).
The Dialing Problem: If a "1" is not dialed before the number then certain carriers like T-Mobile will not know if the number is international. The solution is to make sure that "1" is always advertised if your number uses an Over-Spell number.
Common Errors are T-Mobile says "57 your call cannot be completed as dialed..."
Vanity numbers often use over-spells and carriers like Verizon have no problem when the missing "1" is not used.