I have a Sony SPP-A2780 base unit telephone with one cordless handset.
The cordless handset recently stopped communicating with the base
unit. The instructions said that the handset must be reset, and then
re-registered with the base unit. The reset of the handset worked
successfully (pressing Intercom, Ezy-Access and 1 at the same time),
but the handset will not register with the base unit. To register the
handset, I pressed PGM, and then Intercom, entered the system ID
number, and pressed PGM again while holding the handset within a few
inches of the base unit. The base unit acknowledged the registration
attempt, but both the handset and the base unit showed ERROR on their
respective displays after a few seconds. I have also tried the
registration process after pressing the reset button on the underside
of the base unit, after unplugging the base unit, and after taking the
battery out of the handset, all without success. I received some
advice that I would need to reset the base unit in the same way that I
reset the handset (pressing Intercom, Ezy-Access and 1 at the same
time), but this process doesn’t seem to work on the base unit. Sony no
longer produces this model and although I found some references to
handset registration on the Sony service web site, this problem did
not seem to be addressed.  

Some other information:
>The phone is no longer under warranty. 
>I unplugged the handset battery for at least five minutes and
repeated this a number of times.
>I am trying to avoid draining the base unit battery backup for fear
of losing my directory (and I'm not entirely confident that it will
help).  I may try this as a last ditch effort.
>Sony service has not been very helpful.  I will persist with them if
I don't get an answer here.

This question is a re-post of a previous question.

I am having the same problem and was wondering if you have found an aswer yet?

No - no answer yet.

It's amazing that these are coming up so close together.  This morning
one of my H273 remote handsets did exactly the same thing -- wouldn't
connect to the base and wouldn't register, either.  It must be
something wrong with the transmitter in the handset.  No wonder Sony
got out of the business.

I wish I could help you, but I am having the same problem, started in
the last 3 weeks.  My handset battery died and I got a new one from
SONY (no one else had any) and the handset didn't work.  I tried all
the above with no luck.  Anyone come up with any way to fix there
phones?

SUCCESS!!!!

Ok, I had the exact same problem and this is what fixed it for me:

1st) Reset all the handsets using the intercom+ez access+1 (all at the
same time)

2nd) Reset the base using the same combo (all at the same time again).
This took me several tries to get right. Just keep at it.

3rd) Gather all your handsets into a happy group near the base.

4th) Register the handsets one by one.

This worked for me. Hope it does for you as well.

Siliconjunkie is a genius! 

His solution works perfectly! Apparently Sony does not mention in the
manual or on their website that the base unit must also be reset if
you are replacing a bad handset with a new one.  That was my
situation. I bought an extra handset to replace a dead handset and was
unable to register it.  I reset all of my handsets and was then unable
to register any of them. I was totally screwed! I reset the base and
then was able to register all my handsets. Thanks!