Help no ring tone

[ardesia]

Occasionally my phone just does not ring or indicate when I have a missed call. It is not consistant so this is difficult to diagnose. Last evening my husband was trying to find me in Costco and the phone did not ring on my end - He could hear the rings on his phone. When we finally did meet up we tested it and since it was not working I went to the Verizon booth where they went through several procedures testing it and could not figure out what was happening. They assumed the phone was dead. Later in the evening it started ringing again and working perfectly. I can call out without problems. Any ideas?