Send event based SMS without customer email

Hi, I am trying to send an automated outbound SMS message for a referral program, which is triggered by an event we send from within our backend software. 

In the past, we have been able to use the customer’s email to identify their contact info in Intercom, but in this case, we want to send texts to new users whose email we may not have yet, we will have just their phone number. 

Is there a way to send an event without needing the email field, or a way to workaround this limitation?

Typically we’d send an event with the payload like this: 

const data = {
      eventName: InterCommEventNames.refer_a_friend_invite,
      createdAt: Math.round(Date.now() / 1000),
      email: "",
      metadata: {
        referrer_customer_id: lytCustomer.id,
        referrer_customer_email: customer.email,
        referrer_first_name: customer.firstName,
        referrer_last_name: customer.lastName,
        referral_code: referral.referral_code,
      },
    };

But if I try to drop the email parameter and include phone instead, it gets rejected by the API client for missing parameter.