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
Siemens SL45 GSM mobile
All-O-Matic SL45 DC Slide Gate Opener (Limited Time Sale) – Elite Gates
Battery: 5165 - NOKIA - CEL-57LI
Vetus - Solar charger 45 A, 12/24 Volt, Part No. SL45 – MMarine Online