illumy usernames need to be different from phone numbers when using internet calls (VoIP).
illumy usernames must have at least one letter in them. This is to avoid usernames that are only numbers, dashes, underscores, or dots. These symbols are usually used for phone numbers, not email addresses or usernames.
Using symbols in usernames can cause problems with internet calls. Internet calls need different symbols for usernames and phone numbers to work properly.
Having at least one letter in a username helps tell it apart from a phone number. This makes sure calls go to the right user.