C gets() command is deleting variable data - c

Question

I have to write a program in C for a school project that registers a geriatric client, this is my code:
int position=0, correctData=0, day, month, year, phone, iRS, exercises;
float height, weight;
char confirmation, name[100], address[100], numberToCheck[9];
for(int i=0; Client[i]->name!="NULL";i++){
position++;
if(i==30){
break;
}
}
do{
fflush(stdin);
system("cls");
printf("\n\tINSERT NEW CLIENT");
printf("\nInsert client name: ");
gets(name);
printf("\nInsert client address: ");
gets(address);
printf("\nInsert client birthday(dd/mm/yyyy): ");
scanf("%d/%d/%d", &day,&month,&year);
do{
fflush(stdin);
strcpy(numberToCheck, "");
printf("\nInsert client phone number: ");
gets(numberToCheck); //THIS IS THE LINE ERASING MY ADDRESS VARIABLE
if(isPhone(numberToCheck)==0){
printf("\nNumber not valid");
}
}while(phone(numberToCheck)==0);
phone= validateInput(numberToChek);
do{
fflush(stdin);
strcpy(numberToCheck, "");
printf("\nInsert client tax number: ");
gets(numberToCheck);
if(isIRS(numberToCheck)==0){
printf("\nNumber not valid");
}
}while(isIRS(numberTocheck)==0);
iRS= validateInput(numberToCheck);
printf("\nInsert client height: ");
scanf("%f", &height);
printf("\nInsert client weight: ");
scanf("%f", &weight);
printf("\nInsert the number of exercises made: ");
scanf("%d", &exercise);
fflush(stdin);
system("cls");
printf("Name : %s", name);
printf("\nAddress : %s", address);
printf("\nBirthday : %02d/%02d/%04d", dia, mes, ano);
printf("\nPhone number : %d", telefone);
printf("\IRS : %d", contribuinte);
printf("\nHeight and Weight : %.2fm e %.2fKg", height, weight);
printf("\nNumber of exercises : %d", exercise);
printf("\n\nIs the data correct??(Y/N): ");
scanf("%c", &confirmation);
if(confirmation=='Y' || confirmation=='y'){
strcpy(Client[position]->name, name);
strcpy(Client[position]->address, address);
Client[position]->dayBirth = day;
Client[position]->monthBirth = month;
Client[position]->yearBirth = year;
Client[position]->phone = phone;
Client[position]->iRS = iRS;
Client[position]->height= height;
Client[position]->weight= weight;
Client[position]->exercise = exercise;
correctData = 1;
}
}while(correctData==0);
printf("\nUser registered");
printf("\n\nPress Enter to continue");
system("pause >nul /nobreak");
I receive by reference the array Client[100] so that i can modify its elements.
In the part of the code to check if it's a phone number/iRS number i use a pre-coded submodule to validate it.
The problem I'm having with the code is that it stores the address of the client but when the program receives the numberToCheck to check if it is a phone number the address variable is cleaned, becomes empty and I don't see why.
I appreciate any help and time given to help me solve the problem