Hi, wonder if anyone can help...?

I am in Australia, near Sydney.

I have an Ericsson T28s, of which I am very fond, (in fact I have two).
I have had literally a dozen phones, and have never liked one so much.
So I bought two, just in case one died.

I use an AAPT prepaid SIM card, and everything has been fine, until
now.

Two days ago I missed an incoming call, while away from home.
I was in the room at the time, and the phone didn't ring.

But I heard the beep to tell me I had a message on voicemail.  I called the
caller back and established that he had tried twice and there was no answer,
just a few minutes previously.

So I retrieved my trusty old CDMA, and called my Ericsson.

The usual message appeared on the screen to tell me there was an incoming
call,
but the phone didn't ring, nor did the display light up.

Of course, I checked the profiles, that it was not on "silent" (it wasn't)
and the ring tone was my chosen one, active, on maximum, as usual.

I tried again.  Same deal.  Message "incoming call", no light or ring.

I then RESET the phone to defaults, just in case.

Still no ring...

I tested the alarm, and it worked fine, so I used that next day, no problem.

When I returned home, I found that my OTHER T28s did exactly the same
thing!

I switched my SIM card to two different Panasonic phones.  Rings fine, no
problem at all.  Worked 5 times, no worries.
Tried a Motorola, no worries.

So I switched back to the T28 and it RANG!  But only on ONE call.  when I
tried a second time, it DIDN"T.

I tried the OTHER T28.  Rings on the FIRST call ONLY!  And then ONLY after
the SIM being "conditioned"(?) by receiving a call on a different phone,
which
rings every time...

Incidentally, using my friend's SIM in the T28 cures the problem.
He uses the same GSM network through a different service provider.

Anyone have any idea what weirdness is happening here...

AAPT service operators, of course, have no idea, but grudgingly
offered me a replacement SIM, which will take a week.
I don't want to change providers, as their rates are good.

Thanks in advance,

Michael