How to spy on SMS messages without the phone?

It is possible to spy on SMS messages without the phone, when:

  1. People use the same mobile provider and program that includes a few members (e.g., Verizon Smart Family or T-Mobile Primary Account Holder with a family plan);
  2. Phone numbers are associated with Google Voice ones and SMS can be read on Google Voice account;
  3. Using spyware which works in a stealth mode, records another person’s SMS, and delivers them to a user.