This is Python The program should accept input from the user as either 7-digit phone number...

def convert(n):
if(n=="A" or n=="B" or n=="C"):
num=2
elif(n=="D" or n=="E" or n=="F"):
num=3
elif(n=="G" or n=="H" or n=="I"):
num=4
elif(n=="J" or n=="K" or n=="L"):
num=5
elif(n=="M" or n=="N" or n=="O"):
num=6
elif(n=="P" or n=="Q" or n=="R" or n=="S"):
num=7
elif(n=="T" or n=="U" or n=="V"):
num=8
elif(n=="W" or n=="X" or n=="Y" or n=="Z"):
num=9
else:
num=-1
return num
  
def validate(number):
if number[0]=="-":
print("The first character cannot be a dash")
return False
elif number[len(number)-1]=="-" or number[len(number)-2]=="-" or number[len(number)-3]=="-" or number[len(number)-4]=="-":
print("The last four characters cannot contain any dashes")
return False
  
number=number.replace("-","")
  
if len(number)==10 or len(number)==7:
new_number=""
if(len(number)==7):
new_number="512"
for i in range(0, len(number)):
if(number[i].isalpha()):
new_number+=str(convert(number[i]))
if(int(convert(number[i]))==-1):
print("Number contains invalid characters")
break
else:
new_number+=number[i]
n2=new_number[:3]+"-"+new_number[3:6]+"-"+new_number[6:]
print(n2)
return True
else:
print("Length is not 7 or 10")
return False
  
name=input("Enter your name: ")   
while True:
number=input("Enter 10 character telephone number: ")
if validate(number.upper()):
break
else:
stri =input("Invalid number, Want to try again?or enter quit ")
if(stri=="quit"):
break
print("Goodbye "+name+"!!")

Screenshot of code

Output Screens