we using API with our WHMCS section but to receive alert, do you want to send out bulk email to your customers from admin area ? it may be need some programming and maybe impossible. You should run another script that connect to your WHMCS database and send SMS to all members, however you should check:
if it is a mobile number
if it entered correctly
if it entered in correct international format
what members do you want to receive SMS and more
this need custom programming and it maybe not free.