Tying is illegal in the US. The question is 'is that tying?'.
The answer is: yes. The only way this is not tying is by bundling the service: apple would provide the number for you without charging.
But then, D&B competitors could say this is a trust case.
To really solve this apple must offer a list of companies that offer the solution for whatever numbering problem they seem to have. Or just drop the requirement.