1) I can't say conclusively, until you can report back on what you found with each of your forwarding numbers. Something changed with either the way you are using that phone and its number, or with that number's service.
2) Call forwarding is a service that your mobile or land line service provider would offer. You change the settings by one of two ways:
If the service provider supports call forwarding (some carriers don't, and some charge an optional/extra fee), then you enter commands on the phone's touch-tone keypad. For example, a user has a T-Mobile phone number, and they want to use it as a linked number with Google Voice, such that unanswered or busy calls that reach the cell phone should be bounced back to Google Voice's voicemail. Assume their GV number is 12138675309. They would enter this string on their phone: **004*12138675309# and touch send.
Some carriers provide a web portal for their customers to control these features. Three examples are Verizon Wireless, Ting, and Republic Wireless. In those cases, you can sign into your account with that service provider, and make the settings there.
The reason you need to do this, leads to your third question:
3) If a forwarding phone does have working/paid phone service, but that phone is turned off, or busy, then that phone carrier's voicemail service will immediately answer the call and take the message, thus preventing any of your other forwarding phones to have a chance to ring. If that forwarding phone number was canceled/abandoned (you no longer have service on that number, but forgot to remove it from your Google Voice linked numbers list), then, some months later, when that carrier recycles the number and provides service on it to another customer, THAT customer will be getting your phone calls.
