Unfortunately, I do not know of an Access facility that does this directly. However, I have seen third-party phone tie-in programs with vocoders that can be triggered by another application. Do you have one of those?
If so, you want to issue a command through the Shell function, where one of the shell parameters in the command line is the text you want the vocoder to speak.
Access by itself does not have the facility unless I totally missed it as a new feature for an install option in my Office XP options list. Since Word (at least in Word XP) has the ability to read text to you, it should be possible to give the vocoder something to say through COM / Active-X operation.
The part I don't think is possible is to tie it in with phone software unless the phone software allows it. So your starting point isn't Access, it is whatever you are using for your telephone software. If your phone software isn't integrated with your PC, then I don't see any way to do this.