Buying only one Twilio phone number in Node.JS - node.js

Question

I want to buy one phone number from Twilio which has the same area code as a customer. When I run the code below, sometimes several numbers are purchased instead. This is not ideal as it runs up my costs.
exports.buy_twilio_phone_number = function () {
prefix = "+1415";
return new Promise (function (resolve, reject) {
client.availablePhoneNumbers ('US')
.local.list ({
contains: prefix
})
.then (function (data) {
if (data == undefined || data.length == 0) {
reject (new Error ("couldn't get a phone number"));
} else {
client.incomingPhoneNumbers.create (
{
phoneNumber: data[0].phoneNumber,
}).then (function (new_twilio_number) {
console.log ("new twilio number purchased " + JSON.stringify (new_twilio_number));
resolve ( new_twilio_number);
}).catch(function(error){
console.log ("could not get the number error");
reject (new Error ("could not add phone number"));
});
} // end else
}); // end then
}); // end Promise
} // end function
The function to purchase twilio number is called from a post request (after the user creates an account and hits the submit button). This is the gist of the code.
exports.editPatientPost = function (req,res) {
var form = new formidable.IncomingForm();
form.parse(req, function(err, fields){
// process user's account
buy_twilio_phone_number ()
.then(function (twilio_number) {
if (err) console.log (err);
updateUserWithTwilioNumber (twilio_number, function (err, user) {
if (err) console.log (err);
return res.redirect (303, /'account_created');
});
}) // end then
}) // end form parse
} // end function
I think .local.list is the problem where it calls the .then more than once. How can I ensure only one phone number is purchased?