Hi your Blackberry Flip mobile is locked to network due to this it wont accept other network sim in it .To solve this you must unlock it .There are many ways found to unlock mobile like software ,cable codes etc.... software will damage the OS and this will reduce the signal reception on your mobile so codes are the best and simplest way to unlock it.To get codes visit
classicunlocking.com here they ask the network provider name to which your mobile is locked and the imei number of your mobile. Finally they send you codes to unlock your blackberry flip mobile;.