Microcell stopped working on 2 phones after vacation, still works for everyone else.

If all of the lights on the MicroCell are solid green (Power, Ethernet, GPS, and 3G) and all but two of your phones still connect with no issues, then it's more than likely the phones and not the MicroCell.

Samsung Galaxy phones have a history of not playing nice with the MicroCell because of OS oddities. There is very little that AT&T can do because it sounds like a Samsung problem.

Do you know if the problem phones received some sort of update? Did they vacation in a foreign country where they may have inadvertantly received an update from the local carrier?

It appears that you've done what we would have suggested. The key is the hard reset on the MicroCell. If done properly, that resets the MicroCell back to default settings and once  you restore power, it goes thru the Initial Acitvation process all over again (see Tech Guide for details, link in my sig). If your other phones still connect without any difficulties and it's still the two that don't, it's the phones and I'm not sure what you can do at this point in time. Samsung is notoriously uncooperative when it comes to the MicroCell. And, to be fair, if their phones work outside of the MicroCell range, then as far as they are concerned there's no problem.

They went to Minnesota for two weeks and barely used their phones at all because of the terrible service up there.  There has to be a logical explanation.  I would assume network settings since the phones were maybe searching for signals, but I can not find anything useful.  My Mom claims that most of the time they were gone that a circle with a line through it was what showed for cell signal on the phone.

Well I certainly wouldn't consider Minnesota a foreign country (some may disagree with me though ) but it does sound like something happened to the phones. You would think that they would have received reception in Minnesota as well and not have the "No Signal" icon appear.

I would agree that this doesn't make sense but again I go back to the fact that the MicroCell doesn't indicate anything wrong (the light pattern is ok) and other phones still work with it as before. Maybe Samsung has an idea of why the phones worked before the vacation, there were apparently issues while on vacation, and now they won't work with AT&T's VoIP service where they did before. Maybe a system restore on the phone's is in order.

Nope, the Minnesota thing is normal.  If you live in rural Minnesota than you have a Verizon phone since the AT&T service is abysmal.  I was hoping to avoid restoring the phones by finding an answer here.  I will wait a little while, but total restore is next on the list.  I am aware that the Microcell itself is working properly and I did not mean to suggest otherwise.  I am hoping to find someone that has had this same issue with their phones (any brand) to give me some tips if they were able to correct it, but thank you for your suggestions.

Sorry I couldn't have been of more help. It's just one of those things that seems to happen to Samsung phones more often than others. Something got changed in the phone

Backup the affected phones and do a complete system restore on them.  Let us know if that helps or not.  It's the phones not the Mcell that has changed.