Twilio queue overflow error: how large is the queue? - twilio

Question

Twilio's Message resource has a "status" property that indicates whether a SMS message is "queued", "sending", "failed", etc. If a Message instance failed to deliver, one possible error message is "Queue overflow". In the Twilio documentation, the description for this error case is: "You tried to send too many messages too quickly and your message queue overflowed. Try sending your message again after waiting some time."
Is the queue referenced in error code 30001 an instance of this resource? https://www.twilio.com/docs/api/rest/queue
Or is the queue (in the case of a 30001 error code) something that Twilio maintains on their end? If Twilio does throttling behind the scenes (queueing SMS messages per sending phone number), what is the size of that queue? How much would we have to exceed the rate limit (per phone number) before the queue overflow referenced in error code 30001 occurs?

Answer 1

Emily, message queue is not related to the queue resource you linked to above and it is something maintained on our end.
Twilio can queue up to 4 hours of SMS. This means, we can send out 1 sms per second, if there are more than 14,400 messages in the queue, all the messages queued up after will fail with 30001 error queue overflow and will not be sent. This info is for Long Code numbers. The link above explains processing for other scenarios.
A few suggestions to avoid the error:
Keep messages to at most 160 characters if possible. But if not
possible, calculate how many SMS messages each message will be (if
you are not sure you can always send 1 test message and see how much
you are charged for that message).
Based on the assumption that your messages is 160 characters,
throttle the sending rate to 3600 messages per hour (1 messaage/sec *
60 sec/min * 60 min/hr).
Please let me know if you've got any other questions.

Answer 2

Each of the Twilio phone numbers(senders) has a separate queue in which 14400(4 hr x 60 min x 60 sec) message segments can be queued. 1 segment is sent in one second.
What is a message segment?
A message segment is not a complete message but a part of a message.
Normally SMS is sent in terms of message segments and all message
segments are combined on the user’s mobile to create the actual SMS.
Twillio message segment config:
1 character = 8 bits(1 byte)
GSM encoding = 7 bit per character
UCS-2 encoding = 16 bit per character
Data Header = 6 bytes per segment
Summary: Each character takes 8 bits, If GSM encoding is used, each
character will take 7 bits or if UCS-2 encoding is used, each char
will take 16 bits. In the case of multiple segments, 6 bytes per
segment will be used for data headers(responsible for combining all
segments of the same SMS on user mobile)
Character per Message Segment:
GSM encoding when single segment = (140 char bytes x 8 bits)/ 7 bits =
160 characters
UCS-2 encoding when single segment = (140 char bytes x 8 bits)/ 16
bits = 70 characters
GSM encoding when multiple segment = ((140 char bytes - 6 header
bytes) x 8 bits)/ 7 bits = 154 characters
UCS-2 encoding when multiple segment = ((140 char bytes - 6 header
bytes) x 8 bits)/ 16 bits = 67 characters
Based on what encoding is used(check via Twilio Admin) for your message, you can calculate how many SMS can be in the queue at a time.
References:
https://support.twilio.com/hc/en-us/articles/115002943027-Understanding-Twilio-Rate-Limits-and-Message-Queues
https://www.twilio.com/blog/2017/03/what-the-heck-is-a-segment.html