Even though the 5510 was revolutionary, it technically wasn't the first phone with an MP3 player. The Siemens SL45 introduced this feature almost 4 months before, in August 2001! : r/vintagemobilephones
SL45/SL45i Modifications - Minijack audio, antenna removal, battery flashlight, car adapters and other tricks
Battery - Siemens SL45 User Manual [Page 10] | ManualsLib
BATTERIE NEUVE Compatible Packard Bell SL35 SL45 SL51 SL65 SL81 - 11.1V - 7200mah - Gar 12 mois -
SL45 solar powered street light with pole and LED power choice
Buy Cadex 07-110-8590 | Cadex charger battery adapter. To be used with model V30145-K1310-X136-1 Li 3.6V 550 m ah battery, made for Siemens SL45, SL45i, SL42, 6688, 6686