Despite strict industry commitments to network uptime, engineers have yet to settle on a single, universally accepted method for computing the Network Unavailability Rate (NUR). This paper derives and comparatively evaluates five NUR formulations — the classical uptime ratio, MTBF/MTTR-based reliability model, failure-rate (λ) approximation, Erlang queuing model, and the telecom cell-based model — within a unified analytical-simulation environment applied to LTE/5G networks. A Monte-Carlo framework was constructed over a synthetic topology of 1,000 cells across 50 replications to isolate each model's sensitivity to MTTR, failure rate, and spatial burst failures. Results show the classical model underestimates true downtime by roughly 18%, whereas the cell-based telecom formulation achieves an error of approximately 3% by capturing both temporal and spatial failure dynamics. The MTBF/MTTR, λ, and Erlang models occupy an intermediate error band of 9–12%. These findings point toward cell-level KPI aggregation as the preferred basis for SLA monitoring and reliability engineering in contemporary and forthcoming 5G deployments.