Why Can't I Make or Receive Calls?

Everything else seems to be working fine, even text messages, but after about 7 hours I can't make or receive calls. If I turn the phone off (then on) it's good to go for another 7 hours, but then the same problem recurs. By the way, I've only had this phone for two weeks and I paid good money to have a technician set it up for me. My provider, Bell, is running a remote diagnostic, but in the meantime, I am finding this issue disturbing. If I'm told to do a factory reset, I'll have to spend more $$$$ because I don't feel equipped to run one myself. any advice would be more than welcome.