I assume your "55" is an access code to a specific route data block (RDB). When you dial "55" you directly access the paging trunk. If this is true, then I would assign a specific TARG number (for example, 5) to that route data block. Then, on that specific phone, also put a 5 in the TGAR field. This will restrict them from dialing the paging trunk. Assuming the rest of the extensions are not a TGAR of 5, they will still have access.
Hope that helps,
Richard