What route patterns a phone can use is defined by what calling search space that the device and/or directory number has been assigned with. In the CSS you have a list of partitions and anything in any of these PT's , including RP's is what the device has access to use.
As Nithin pointed out, you can achieve this by creating a new partition, and placing it in a new Calling Search Space. Then, assign your newly created partition to the route pattern in question. After that, assign your newly created Calling Search Space to the phone that you want to reach the route pattern.
Also, to give the phone the ability to access other patterns, you can simply add the existing partitions of those other patterns to the new Calling Search Space.