This a code for a near problem. In works with serial an strings in program memory.
If you need to use a dynamic update you must use a constant length or calculate the vector to point at the beguining of string when you recive it.
....
MOV A,#MSG_NUM
LCALL TB_MENSAJE
....


TB_MENSAJE MOV DPTR,#MENSAJES
RL A
CLR C
ADDC A,DPL
MOV DPL,A
JNC T_M_1
INC DPH
T_M_1 CLR A
MOVC A,@A+DPTR
MOV B,A
CLR A
INC DPTR
MOVC A,@A+DPTR
MOV DPH,B
MOV DPL,A
PRINV CLR A
MOVC A,@A+DPTR
JZ F_PRV
JNB TI,$ ;Change this for LCD
CLR TI ;
MOV SBUF,A ;
INC DPTR
SJMP PRINV
F_PRV RET

ORG MENSAJES
DW MEN1,MEN2,MEN3,MEN4...

MEN1 DB 'Msg1',13,10,0
MEN2 DB 'Msg2',13,10,0
MEN3 DB 'Msg3',13,10,0
MEN4 DB 'Msg4',13,10,0
...