Firstly, make sure you have purchased a phone number. Please use this guide if not: https://help.reffle.co/en/articles/10502307-business-verification-phone-number-setup-guide
To send Bulk SMS to all customers:
Select the contacts you want to send the SMS to (you'll have a 'select all contacts' button) and click 'send sms' in the top.
You'll get promoted with this screen:
Select the phone number you want to send it from, type your message and decide whether you want to send it all now, schedule it or drip feed it.
The same goes for smart lists. I'd really suggest breaking down clients by smart lists so it's not so generic.