In order to mitigate theft and other fraudulent activity, newly purchased devices are “locked” to work exclusively on the Verizon network. We have separate device unlocking policies that cover postpay and prepaid devices, as well as special rules for deployed military personnel.
www.verizon.com
If it isn't unlocked after that period. Try contacting Verizon support. Ask for carrier unlock during the phone prompts. You'll need the IMEI number and phone model. If you don't have an existing Verizon account. I couldn't answer if they'll unlock it for you.
You could also try one of those carrier unlocking services. Although I could not say which are and which are not legitimate.