Height is your enemy. Below is a copy of the information found on my web
page in the FAQ concerning this issue. I've copied it to save having to
rewrite everything in this reply:

Most people experience very poor service when using phones in tall buildings
(like apartments and office towers). There usually isn't too much of an
issue below the 10th floor, but above that elevation it is rare that you
will get decent service on any provider. The problem is different for CDMA
phones and for TDMA/GSM/iDEN phones, but the end result is identical.

Let's start with TDMA/GSM/iDEN phones, and see why being up high is such a
problem. All of these technologies work on the principle of channel reuse,
which means that any given channel is reused as frequently as possible
throughout a geographic area. However, the closer these sites of the same
channel are, the greater the chance that interference will occur. We call
this co-channel interference because the problem comes from an unwanted
signal on the same channel. Network engineers are very cognizant of
co-channel problems, and so they tune their networks so that sites working
on the same sets of channels don't carry far enough to cause interference to
one another.

Once you go up in a building however, you are getting above all of the
geographic and manmade obstacles that limit the range of a site. Once you
are up high enough you can potentially receive signals from every single
site in the city. You pick up 5 or 6 sites on the same channel, and the
level of interference may even get high enough that little or none of the
desired signal can be picked out from the mess.

With CDMA the problem of co-channel interference isn't a direct issue, since
CDMA is one huge co-channel network. All phones and all sites work at the
same frequencies. Individual users are selected from this hodge-podge of
signals based on their encoding (thus the term CODE Division Multiple
Access). At first glance it would seem that CDMA would be immune to such
problems. However, CDMA cannot support an infinite number of calls on the
same frequency. As the number of calls increases, so the overall background
noise goes up, until it reaches a point where none of the signals can be
discerned.

This type of interference in CDMA is known as channel pollution, and moving
up in a building simply exposes your phone to more and more of the sites in
the city. The more sites you get exposed to, the higher the pollution level,
and thus the greater the damage to the audio.

For both CDMA and TDMA/GSM/iDEN phones however, there is one way that a
phone can work in a tall building. It must be close to a site, which would
then provide a dominant signal that can rise above any possible interference
from distant (and much weaker) sites. However, this close site would need to
be at approximately the same elevation as you were. If it is too high above
you, or too far below you, the signal strength of that site will be
insufficient to be dominant. An indoor repeater would certainly serve this
purpose, and some office buildings may indeed employ indoor repeaters to
provide good coverage, even at high elevations.

My UK Triband T200 worked fine on top of the CN Tower in Toronto