Does SIPSTACK Check If Phone Numbers Can Receive SMS?
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Does SIPSTACK Check If Phone Numbers Can Receive SMS?
Section titled “Does SIPSTACK Check If Phone Numbers Can Receive SMS?”SIPSTACK performs basic phone number validation as part of every outbound SMS request. The goal is to reject messages to numbers that are clearly invalid before attempting delivery, saving you from being charged for undeliverable messages. However, SIPSTACK trusts that you know more about the recipient’s number than the system does, and validation only rejects numbers that are definitively incorrect.
Number Validity Check
Section titled “Number Validity Check”When you submit an SMS via the SIPSTACK API, SIPSTACK validates the To phone number:
- Invalid number: If the number cannot possibly exist (malformed, wrong length, invalid country code), SIPSTACK returns an HTTP 400 response with error 21211, and the message is not sent.
- Possibly valid number: If the number could exist, SIPSTACK passes it to the carrier and records the message with a status of
queued.
Mobile Number Check (Country-Dependent)
Section titled “Mobile Number Check (Country-Dependent)”Depending on the country, SIPSTACK may perform an additional check to confirm the To number is capable of receiving SMS.
| Country Type | Behavior |
|---|---|
| Countries with a defined mobile range (e.g., most of Europe and Asia) | Numbers outside the known mobile range return HTTP 400 with error 21614 |
| Countries without a defined mobile range (e.g., Canada, US, most Caribbean nations) | SIPSTACK attempts delivery to any valid number — there is no reliable way to distinguish mobile from landline |
Best Practices
Section titled “Best Practices”- Collect phone type at opt-in. Ask recipients whether they are providing a mobile number, and confirm consent to receive SMS.
- Only message opted-in users. Sending to people who have not consented increases opt-out rates and spam complaints, which can lead to carrier filtering.
- Validate numbers before storing them. Use SIPSTACK’s Lookup API (if available on your plan) to verify number type before adding to your messaging list.