Most handsets have security measures built into their firmware that protects them from repeated attempts to guess the unlock code. After entering more than a certain number of incorrect codes the phone becomes "frozen". This is a state where the phone will display a security message that the phone needs a service. Older phones could no longer be used at all at this point, however modern smartphones often keep working with the original SIM but require extra work to then unlock them correctly. In extreme situations physical access to internal hardware via in-circuit debugging may be utilised (for example, via JTAG headers on a circuit board). Such access may be required to modify initialization software used for booting.