Apple Business Manager OTP codes not coming through

I'm trying to look into Apple Business Manager for my company (we have 11 Apple IDs) but when I try to create a new account (it's says not to use an email address not already associated with Apple) but when it comes to getting the OTP codes through to the mobile number they are not coming in. Even if I try the Call Me option, I don't get the call.

I'm definitely entering the correct mobile number, it's a UK number.

I was having issues with getting OTP codes, after taking to the mobile provider they suggested disabling RCS and the Spam Protection and the ones that were not coming through started to come through except for Apple's. Our office mobile is a Motorola running the latest version of Android and we use the default Messages app (Googles).

Any thoughts as to why?

I could use my personal mobile number which is already associated with AppleIDs but I'd rather not if I can.