Not receiving international OTP after upgrading to IOS 18

Not receiving OTPs from outside the country(Canada) after upgrading the IPhone 13, IPhone 13 Pro and IPhone XR to IOS 18. However, when registered another phone having an Indian SIM (IPhone 7.1) running on Release 15.8.3 on the same website, the OTPs were received successfully. These 3 phones were receiving the OTPs before the upgrade and there were no issues. Are there any known issues with IOS 18?

-completed standard troubleshooting steps such as keeping Aeroplane mode ON and restarting the phones and toggling MMS messages on the messaging app did not help.

Any help would be appreciated.