when talking to a caller, anolther call came in causing a beep noise but the battary area got suddenly hot.
Hi there. When electrical current flows from the battery to the phone it creates heat due to resistance in the wiring and circuitry. The amount of heat generated when the phone is not in use is negligable, however, when you are using the phone you are consuming more power and, thus creating more heat. Nothing to worry about unless you notice the battery bulging.
Source: Link