Yes—it's feasible in principle, and variants have already been built. The main challenge is packaging: a longer-equivalent lens generally needs more optical path length than a typical wide phone camera, so fitting it into a thin phone is harder. That’s why designs may use folded optics (for example, mirrors/prisms/periscope-style layouts) to fit a more complex telephoto path inside the phone body.

A true telephoto module is preferable to digital zoom because it preserves image quality better, but it adds cost, space, and design complexity. Hybrid devices like the Galaxy S4 Zoom showed that optical zoom in a phone-like device is possible, though with size trade-offs.

Another concept is to keep the device thin by changing sensor strategy rather than using two separate lenses—for example, pairing different sensor sizes or pixel densities to cover wide and tighter fields of view. That can reduce protrusion, but it also increases complexity and may not fully replace the benefits of dedicated optics.

So the answer is: it’s not impossible, and manufacturers’ choices are mostly about thickness, cost, and product design priorities rather than a fundamental optical barrier.