I want to get new IPhone 7's for myself and another family member for Christmas. We both have iPhone 5s and need to upgrade. My problem is cel service in my area is bad (phone calls between us always go to voicemail). What I'm asking is which type of iPhone should I buy (AT&T, Verizon, Sprint, no-sim), if I haven't decided on my service yet? I'm still surveying my friends on what they have that works. Any help would be appreciated. Thanks.