usually in these cases where the CPU is so slow, it simply doesn't have the bus bandwidth even to feed an LCD. so the LCD is an SPI device or 8080 MCU compatible memory-mapped peripheral, that has its own internal SRAM as a framebuffer. a good example is the HX8357D (now obsolete sadly) https://github.com/torvalds/linux/blob/master/drivers/video/...

technically speaking it's perfectly possible to use an 8mhz 8-bit arduino-style processor to make a phone, if all you want is calls and SMS! the processor inside the actual GSM/3G modem is far, far more powerful than that.

i added up the number of actual processors in a smartphone once: it was insane. DSP in the Audio IC. ARM core in the GPS. ARM core in the 3G Modem. ARM core in the main processor. 8051 core in the capacitive touchpanel controller. the list just goes on and on!