You will need to use feature access code *12 to change coverage. When you look at the station set up (display station ****) you will see fields marked "coverage path 1" and "coverage path 2". You would usually assign the first coverage path as coverage path 1.
To override the coverage, from the phone in question dial *12 2. This will switch to "coverage path 2". If there is no coverage entered the calls will not forward. To turn coverage back on dial *12 1.
You could set the feature codes as abbreviated dialling and assign them to phone buttons if you want.
Finally ensure that the COR on the phone is set to allow changing coverage. Use command "display COR *" where * is the number, and look for the entry "can change coverage" if this is set to y, the user can use feature code *12. If it is set to no you will need to change the entry using the command "change COR *".