You know, it's been so long since anyone asked about actual phone bills (rather than internet calls like Skype), that I'm not sure.
What you need to show is regular contact between you during the length of your relationship, so it might be best to include just one or two sheets per month showing some of your calls/texts to each other.
It really depends on how long a period of time you need to cover and how often you call each other. You don't want to be including pages and pages, so just a selection spread over your relationship should be fine (say you'd been together 6 months, you might want to include 2 sheets per month, but if you're covering say, 6 years, maybe 1 sheet per every couple of months).