解决此问题的方法是在package.json中添加twilio作为依赖项，同时确保在firebase函数的javascript文件中正确引用twilio库。

具体代码示例如下所示：

package.json:

{
  "name": "my-firebase-functions",
  "dependencies": {
    "firebase-admin": "^8.0.0",
    "firebase-functions": "^3.0.0",
    "twilio": "^3.0.0" 
  }
}

index.js:

const functions = require('firebase-functions');
const twilio = require('twilio');
const accountSid = 'YOUR_ACCOUNT_SID';
const authToken = 'YOUR_AUTH_TOKEN';
const client = new twilio(accountSid, authToken);

exports.sendSMS = functions.https.onRequest((req, res) => {
    const phoneNumber = req.body.phoneNumber;
    const message = req.body.message;
    client.messages.create({
        to: phoneNumber,
        from: 'YOUR_TWILIO_PHONE_NUMBER',
        body: message
    }).then(() => {
        console.log('SMS sent successfully');
        res.send('SMS sent successfully');
    }).catch((error) => {
        console.log('Error sending SMS:', error);
        res.status(500).send('Error sending SMS:', error);
    });
});