Answer is you cant !! Unfortunately you are governed by the cell site which sends commands to your mobile phone instructing your phone to either down power or up power on RF out
The second problem that you have is not just down to cell site location but also down to atmospherics, buildings etc, drop out areas , weather changes
Your phone has to be sensitive to a certain degree in order to receive broadcasts from the site If that is insufficient to start with and you cant get reception your phone will not be able to register on the site and so you obtain what is called no service available
If you did try to increase the power you would certainly get into stacks of trouble because they would pick this up straight away meaning you would be facing prosecution for interfering with BT telecoms That’s a heavy duty fine and possible imprisonment and of course these days they confiscate your equipment
You could however use what the call a passive antenna into the building which picks up what ever signal is their and propagates the signal into the building Such methods have been used in the Dartford tunnel
The only other suggestion that I can make is develop a software interface for a PC remotely attached to your mobile phone which is networked to that in your office and then use that application to either call or send sms Other than this its down to commercial products that are available on the market which enable you to send free sms with replies
Such a site can be found at this address
http://www.cbfsms.comAlternatively think about two way radio coms or trunked radio
Or Skype
http://www.skype.com/Mark
www.harrington.force9.co.uk