The phonebook is in the save file at 0xC0EC - 0xC136. An empty book is filled with 0xFF, a full book can be filled with the following values
0x4B and any higher value results into a duplicated entry of the mother.
If the first entry is empty (0xFF) the game will crash.
A trainer will get added again when you battle him after he was removed.
Fill Phonebook with non-Trainer numbers only (Press L+R)
94000130 FCFF0000
62111880 00000000
B2111880 00000000
D5000000 00000000
2000C0FC 00000000
2000C0FD 00000001
2000C0FE 00000002
2000C0FF 00000005
2000C100 00000006
2000C101 00000007
2000C102 00000009
2000C103 0000000F
2000C104 00000018
D5000000 000000FF
C0000000 00000041
D8000000 0000C105
D2000000 00000000
Edit: if any important person is missing or if any person that isn't useful gets added with my code, let me know and I will fix it!