Allow us to preface this answer by stating for the record that we are not 'phone techs, nor experts on wireless communication, but we'll share what insights we have and if anyone out there cares to chip in with relevant extra info, we'll happily add "Updates" to this answer.
Bad cell-phone service in casinos has been an issue since, well, as long as we can remember and, presumably, since cell phones were invented. There are solid reasons for that which have nothing to do with gambling. Two of the things that are the enemies of good mobile service are: a) heavy network usage, which is an occupational hazard in a city whose economy is based on tourism and conventions (i.e., millions of people away from home needing to conduct personal/business calls) and b) large/tall buildings constructed from steel columns and thick concrete, neither of which is a friend to radio waves.
In Las Vegas, there have been some "political" issues, too. For example, when the futuristic CityCenter metropolis debuted, it was praised for all its forward-looking eco-friendly initiatives, but everyone agreed that cell-phone service there sucked and, with reference to the above paragraph, we can understand why. The powers that be there were quickly made aware of the problem and reacted by installing what was at the time, and may well still be, the largest distributed antennae system in the world. However, all the antennae in the universe won't help you if you don't have agreements with the applicable cell carriers to broadcast their services and for awhile, neither AT&T nor Verizon signed up, meaning their customers experienced even worse service, with many people finding it impossible even to send or receive a text.
That situation was finally rectified, while other Strip casinos have attempted to improve service by installing base transceiver stations, located either in their buildings or in mobile external trailers, to boost signals. We can vouch for the fact that cell-phone service is a heck of a lot better today than it was a decade ago, when it was virtually impossible to make or receive a call inside a Las Vegas casino.
Some googling turned up additional information that might be of help, including reading that mobile-phone reception is likely to get worse the higher up in the building you go, due what's known as "pilot pollution." Here's an explanation from a poster on a Sprint User forum, who wrote: "The higher you go, the more noise pollution you get with the signal. The phone starts to pull in stray signals from too many different PN offsets, causing the phone to bounce around and not know which one to land on. Usually above 10 floors this starts to kick in."
The best advice we can offer is to stay on lower levels if you want to make or receive a call, and get as close to the exterior wall/doorway as possible, to minimize internal interference from the building structure and all the electronic devices a casino-hotel is home to. If anyone out there has more to add, we're all ears (seeing as we're not able to receive any phone calls...)
