Twilio not delivering sms messages to some users

Question

I set up a scheduling website at work which uses Twilio to send out sms messages with each person's next day assignment information as well as a link to the website where notes and everyone's next-day assignment is posted. Occasionally colleagues complain that they haven't received messages. Recently the problem has gotten much worse though is to be limited to people with AT&T and Sprint. People with Verizon and TMobile are get their messages without a problem.

A few more facts:

• All of my code works well in testing and is consistent with Twilio's instructions.
• Twilio's logs list the messages as having been sent.
• The people who's sms messages are not delivered tend to be at the end of the alphabet

Anyone have any ideas what might be going on?

Answer 1

SMS messages the twilio logs show as sent which never arrive at the recipient's phone may be filtered by cell phone carriers as part of an effort to reduce spam sms messages. A website that sends out a batch of messages like the scheduling website you (I) describe may be particularly susceptible to such filtering since spam is also sent out in batches. The fact that recipients at the end of the alphabet tend to be blocked rather than those at the beginning might reflect an algorithm which flags earlier messages as suspicious and blocks subsequent ones that appear similar in that they contain the same website link.

In the past I was able to solve this problem by adding a 1 second delay between messages. That worked fine for my purposes. My site sent out the messages as a background job and with the 1 second delay about 25 sms messages get sent out over 2 minutes.

More recently AT&T and Sprint started blocking many more messages, though not all. At Jan 11, 15:04 PST, Twilio's status was aware of the problem and noted:

Identified - Messages with hyperlinks to goo.gl are being filtered on AT&T and Sprint. We are working with these carriers to address this filtering.

I was able to get around this problem by creating a shortened url using Bitly instead of Google's url shortening service.

Of note, Twilio is a great service but in the past their support folks were unaware that spam filtering could be blocking some of my sms messages. That's my prime motivation for posting about this issue here.

It seems that sites like mine can get caught in the crossfire between sms spammers and wireless carriers attempting to block them. Hopefully some day such sms spam filtering will get more sophisticated and stop blocking the messages sent by sites like mine.

I would love to hear if anyone with more sophisticated insight than mine has any comments.