What board?
If you are talking about the standard boards like the Uno, Mega, and variants of such, they don't use timers for Serial. They use the Hardware UART interrupts - TX complete, and RX complete (cant remember the exact names).
If you look in the datasheets, there are about 26 different interrupt sources (for the uno), some are from timers, and some are from other hardware peripherals (UART, SPI, I2C, ADC, EEPROM, etc).