Lets say my phone number is (123)456-7890.
Should I pronounce it as:
いちにさん よんごろくの ななはちきゅうゼロ
いちにさんの よんごろくの ななはちきゅうゼロ
Native speakers use the 「の's」 most of the time. Without using them, the phone number will not have the rhythm that we expect it to be read out with. It just will not sound "right" without.
For keeping that rhythm, it is also important to know that we intentionally lengthen the syllables of 「に (2)」 and 「ご (5)」 to 「にー」 and 「ごー」, respectively, to produce the same "double-length" as the other digits that are originally double-length.
Thus, the number "(123)456-7890" is pronounced: