Think about; what if you don't know the email address of the person whom you'll send an email to! Will you be able to send email? No you'll not. Therefore if you don't know the phone number of the person, you will not be able to send email via Mailcell.
Answer is already within your question. People has 2-3 email addresses and phone number or numbers. Why we don't leave those habits and gather them all in 1 phone number :-)
But you're right to be a competitor of those big companies.
@KVolkanGunduz Hey, thanks for your reply. You may find it hard to change such an ingrained culture of people running multiple email addresses for different purposes. Also, if I know your phone number, can't I just ask for your email address?