The Siemens optiPoint 500 phone comes in a number of models, here are some details of the ”economy” and ”advance” models.

• LCD, economy 2 rows of 24 alphanumeric characters.
• LCD, advance 2 rows of 24 alphanumeric characters with amber LCD backlight.
• Macronix 29LV400BMC 4 Megabit (512 K x 8-Bit/256 K x 16-Bit) CMOS 3.0 Volt-only Flash Memory
• Infineon INCA-P, PSB 21493 - Infineon Codec with UPN Transceiver and Embedded Microcontroller for PBX phones

It uses a 24 character by 2 line LCD display.

• Siemens optiPoint 500 service manual.

If any referenced page no longer exists, try looking for its URL on http://archive.org.