As someone with a weird sibilant that doesn't seem to compress well, I want to say that it goes across as "sɪkɪks" and I got used to saying "double six" on the phone.

So I would say "seven nine double six", which is another problem if I'm talking to an American.

This applies to GSM digitization and other "regular phone" compression, the newer computer calls have been better at taking the words.