Phone was about 2 weeks old, and 2 days in a row I wasn't receiving calls or texts. I would get voicemail notifications, and texts hours after they were sent to me.
I took the sim and put in my old phone, rang immediately. Put sim back in Tilt, phone wouldn't ring.

After all the issues I had with the 3125, I decided to return the phone and got a Palm Centro.

What software do you have installed on the phone? Is this "stock" Windows Mobile? A custom ROM? Have you reported the issue to AT&T?

There are many possible causes, so more info is needed.

It was new out of the box, the only thing installed extra was Kaiser Tweak on the advice from this board. No other changes on my part, and the problem was completely spontaneous.
As I mentioned, since I had problems with the 3125, which was the reason for the new phone in the first place, I wanted to take care of this before the 1 month period was up.

I was a little wary of the mechanical endurance of the sliding keyboard anyway. And even though it's been years since I owned a Palm device, I prefer the stability of Palm over Windows.

This occurs spontaneously? Or it never rings?

Does a soft reset fix it?

Occasionally, WM has an odd registry setting or something get corrupted, so you might do a factory reset since you haven't installed software of your own and it won't be hard to re-partner. See if you still have the issue after that.

But you're the first to report this as a problem that I'm aware of.