No incoming texts

First HTC Aria I got in October-really like the phone, easy to use, few problems...until about March.  All of a sudden I stop receiving incoming text messages of any kind.  Problem resolves (temporarily) when I pull the battery and restart the phone.  I never get the messages that were sent to me, even after I restart the phone.  (Unlike when if the phone is just off, I will get the messages after turning back on)  Beginning of June, it's become an every day issue, so I go to AT&T store.  The guy puts in a new SIM, and 'resets' the phone to the network.  Works fine...till the next day, then the same thing.  So, I call the warranty exchange number, get new (refurb) phone on Saturday.  Works great, not a single issue...until today.  Any ideas?  I have not restarted the phone, so that they can 'test' it again and see the issue for themselves again.  Will they warranty me out...again?  This is really frustrating.

It's been an ongoing issue for several months now, it started out happening with several days to a week between, to daily and even multiple times a day.  I went to the AT&T store after work today-

Spent 40 minutes in the store, the guy there was on the phone with Technical Department for 30 minutes.  Looked at some settings, turned the phone off then on...and of course it gets messages again.  An hour later, no more incoming messages.  So...I call Customer Support, talk to CS person for 28 minutes...she transfers me to Tech Support, spend 20 minutes there...get transferred to warranty department, another 16 minutes there.  They looked at account settings on their end, they had me look at settings on the phone, they can't find anything that would be causing the issue.  They are sending me another exchange.  So, is there any limit on the number of warranty exchanges before I get a different phone or something?