The standardized conventional format used to write telephone numbers should include the use of dashes within the number. For example, a telephone number should be written like the following: 1-310-234-5678. The use of symbols other than dashes (such as commas, periods, or blank spaces) would tend to contribute to greater confusion due to the use of these punctuation marks for other grammatical purposes.

Commas (1,310,234,5678), if written sloppy, may be misinterpreted as the number one. More likely, the use of commas as place holders in larger numbers may create initial confusion as to whether the telephone number is merely a large number, albeit with commas written in the wrong locations.

Using periods (1.310.234.5678) may also increase confusion because of their conventional use to end sentences. Periods signify the end of a complete thought and their use in telephone numbers would be inconsistent with their original meaning. All the numbers of a telephone number are all directly related with each other.

The use of empty spaces (1 310 234 5678), like the spaces between words, may also increase confusion because the numbers don’t immediately seem to be directly related to one another.

Colons (1:310:234:5678) may be a proper punctuation mark to use, but they may be slightly more confusing than a hyphen because their two dots require more lifting and movements when using a writing instrument than would a single hyphen.
Hyphens seem to clearly imply a direct visual relationship between all numbers in a telephone number sequence while at the same time creating enough spacing within the number sets to make them easier to read.