How a phone hands your call to the next tower without dropping it

This is less a set of instructions about cellular handover than an argument, and it is worth saying so at the start.

The phone is always measuring

While connected, a handset measures the signal strength and quality of its serving cell and of neighbours the network tells it to watch. It reports those measurements back on a control channel, continuously and at very low data cost. The network uses them to decide when another cell would serve you better, which is why the decision is not made by the phone.

Thresholds include hysteresis so that a marginally better neighbour does not cause constant switching back and forth.

Handover is arranged before it happens

The network instructs the target cell to prepare resources, then tells the handset to switch at a specified moment. Because the target is ready in advance, the interruption is typically a few tens of milliseconds and inaudible.

Mechanically, if the handset loses the serving cell before the instruction arrives, there is nothing prepared and the call drops. That is why calls drop in tunnels and lifts rather than on open motorways at high speed.

Cell size trades capacity against handover frequency

Small cells serve fewer users each and therefore give each more capacity, at the cost of more frequent handovers while moving. Large cells reduce handovers and concentrate more users onto shared capacity.

Under load, operators deploy both, with large cells for coverage and small ones for capacity in dense areas. A fast-moving handset in a dense small-cell area generates a great deal of signalling, which networks manage by preferring larger cells for fast movers where possible.

Falling back between technologies

When a newer technology loses coverage, the handset must fall back to an older one, which is a slower and more disruptive transition than a same-technology handover. Voice calls carried over the data network need a defined path to fall back to circuit-switched voice where data coverage ends. That fallback is a well-known source of momentary silence and occasional drops at the edge of coverage.

Where voice over the newer network is fully supported, the fallback disappears and calls become noticeably more robust.

Why the bars mislead you

The indicator usually reflects received power, which says nothing about interference or how many other users share the cell. Full bars in a crowded stadium describe a strong signal on a saturated cell, which is why nothing works there.

The short version: signal quality measurements, available in diagnostic screens on most phones, distinguish these cases directly. A weaker but cleaner signal frequently outperforms a strong noisy one.

Figures here are typical rather than guaranteed — check the spec sheet for your part.

What helps in a bad spot

Moving a few metres, especially towards a window or away from metal and concrete, changes reception more than any setting. Calling over Wi-Fi bypasses the radio problem entirely and hands over to and from cellular on many networks. Signal boosters are effective and are regulated in most countries, so an unapproved one may be illegal to operate.

Locking a phone to an older technology occasionally stabilises a marginal location, at a cost in data speed.

The takeaway

Handover is planned in advance. Calls drop where the plan has no time to be made.

Once you know what it is trading away, the design stops looking arbitrary.