If you are periodically out of cell phone range or have no smartphone, easiest is to purchase and use a Yubikey device.

Yubikey 4, Yubikey 4 Nano and Yubikey Neo will all work with NetID+.

For enrollment check: UA IT NetID+ (Plus) Enrollment Using Yubikey

For login check: UA IT NetID+ (Plus) Login Using Yubikey
 
Instead of using a Yubikey, you can generate bypass codes to use as a second factor of authentication and keep a printed copy of them in your wallet. You will receive 10 codes at a time; remember to cross them out as you use them since each code can only be used once. If you trust the computer you are logging into, you can use the Remember me for 30 days checkbox.

For downloading: UA IT NetID+ (Plus) Bypass Codes