In an increasing number of places, you don't (except perhaps by forcing or convincing someone to let you use their identity). Where I'm at right now, receiving SMS or making phone calls require a subscription with photo ID and physical address - prepaids and travel SIMs are data-only these days. The latter ones also require photo ID for service activation.
But why? It's not anyone's business but my own. If a snotty call phone rep insisted on a treating me like a teenager at a bar they would be some talking and not from my mouth.