The recent efforts of extending cellular technologies to unlicensed spectrum have led to concerns about the coexistence between cellular and incumbent systems. To harmonize coexistence, regions including European Union and Japan require that a radio equipment operating in unlicensed spectrum shall perform listen-before-talk (LBT). In this paper, we study joint listening, probing, and transmission strategies for a frame-based equipment (FBE) in unlicensed spectrum. We first investigate throughput optimal strategy and find that the optimal rule for an FBE is to transmit whenever allowed by the regulation. We then turn to nominal throughput optimal transmission that takes into account transmission costs such as power consumption. We find that the nominal throughput optimal rule is a pure threshold policy: The FBE should stop listening and transmit once the channel quality exceeds an optimized threshold. The optimal threshold can be found by solving a fixed point equation, but the fixed point equation in general does not admit a closed-form solution. We then derive a lower bound and an upper bound on the optimal threshold. We further devise an iterative algorithm with convergence analysis to compute the optimal threshold. Our results shed further light on LBT strategies for radio equipment operating in unlicensed spectrum.