Dear Linn:
Now I can better understand what you mean. As your cell phone can operate under two different systems, its "combo" antenna was certainly designed to function at both frequency bands.
A cell phone, however, operates in a network of base stations. The hardware is so designed that it can function if your cell phone is near one or more base stations, the software is controlling the flow of data and information transmission.
The complete system requires many conditions to be met to function properly. If one of such conditions is not met, the software will not allow the system to function. For instance, the signal power level is important, but if the signal can propagate by "multipath", via two or more separate ways, at one of the receivers the data stream can be distorted and you will be declined from communicating. When you walk or travel, the system is smart enough to enable you to communicate, but interruptions happen if the software is "not satisfied". So the system is defined not only by antenna performance but by many other components, hard- and software.