As I understand it, you want to keep the threshold as low as possible until you've linked the name to the right person, and then obtain a warrant.
As for linking the person to a telephone number, let's say I call you first. You ask me if Claude DeBellefeuille is a customer of the provider. The provider says yes or no to you. If they say yes, then you know my telephone number but not my name, and you continue investigating. However, by that point, it seems to me that you already have a pretty good idea.
If I add that you must have a good reason to continue the investigation—by having “reasonable grounds to believe”—are there currently any investigations where you tell yourself that if I don't give you that information, the investigation will end there?
I think you still have opportunities to obtain the information based on “reasonable grounds to believe” rather than “reasonable grounds to suspect”. Right now, you need “reasonable grounds to believe”, so you want to lower the threshold.