HTC one X plus stops receiving calls

I'm on my 2nd HTC One X plus in less than 2 weeks.  The 1st phone worked out of the box, then 4 days (could have been sooner) inbound calls stopped reaching the phone.  Callers got ringing then a switch to a busy signal or a message that call couldn't be completed.  Nothing reached my voicemail and the phone showed no missied calls or voicemails.  AT&T switched out the sim, the phone then worked for 2 days and again stopped receiving calls.  I could receive texts and call out.  Went back to ATT, received a new phone that worked for 1-1/2 days and now today is not receiving calls.  It either rings (8-10 times), then goes busy, or rings 2-3 times and gets a "unable" to connect message.  I really like the rest of the features of this phone, but if it won't receive calls it does me no good.  Is anyone else having this problem or had it and found a solution before I return it again?  Thanks.