One way to unlock your iPhone 6 Plus, as of June 2015, is using the unlock service at MyIMEIUnlock.com. An alternative unlocking service for iPhones is Keys2iPhone.com.
To unlock from MyIMEIUnlock.com, first enter the carrier to which the phone is locked. If you're not sure of the carrier, use MyIMEIUnlock's instant carrier checker, whose link is at the homepage. Next, enter the phone's IMEI code, and then pay the required fee either via PayPal or by credit card. After this, wait for a confirmation email.
To check whether the phone has unlocked, enter a non-official SIM card into the iPhone, then check if the phone gets a signal; if it does, it unlocked successfully. If you are asked for activation, connect the iPhone to your computer via USB, and then sync the phone with iTunes; you should get a congratulations message on iTunes, stating that the phone has been unlocked. If there is a problem with getting a signal or with the activation process, install the latest iTunes, backup data in the iPhone, and then update the phone's iOS firmware to the latest version.
At Keys2iphone.com, enter the phone's model, select the network it's locked to, and then click on Proceed to check if it's unlockable. If it can be unlocked, enter the IMEI, pay the required fee via PayPal, and then wait for a confirmation email.