We are currently aware of an issue with receiving messages on some phone numbers using the O2 UK network (which has been happening for some time).
We have identified the issue being on the operators side and are waiting for a response from them.
We will update as soon as we have more information. Apologies for this issue.
We have now manually switched the roaming network for the affected SIM cards, this should alleviate the issue for now. The messages should now be coming through.
We will continue watching this issue, if you are still experiencing issues please contact our support and we will do our best to help you.