Your suspicion is correct. Your number gets out there because you give it out, and then 10 years later the company goes under and liquidates their assets which includes a load of customer phone numbers; or because you give it out when you open a credit card or buy a mortgage or car, and you legally can't limit the sharing of your personal info to third parties, who can share your personal info as widely as they want... point is, not having a phone number will be tantamount to not having a social security number in many situations.