When placing calls to any recipient using our Voice APIs, you will need to ensure the phone number is formatted correctly.
For mobile recipients, you must add a "9" to the number after the country code, i.e. 5491123452345. Without this the calls to mobile numbers will fail with a SIP response of 404, 484 or 500 depending on the network. This is different to SMS, where you should not include the 9.
The "9" must not be inserted for calls to land line recipients, or the call will fail with a SIP response 404, 484 or 500. i.e. 542234563456.