Once the quote is sent, the chat within the Opportunity will become available, which you can use to send a message to the Customer. The messages written in the chat will only be visible to the Customer.
If the Customer has shared their phone number, you will be able to view it in the Opportunity details, but only after sending the quote.
We always recommend contacting the Customer by phone as well, and if they are not available, sending a WhatsApp message or SMS to inform them.