gantshill
It is assumed that you are operating outside of controlled airspace.
It's not "RT" its "RTF".
There is no standard phraseology. If you intend descending below MSA/terrain safe level request a Traffic Service, would be advisable to tell ATC that you are/will be descending. ATC may decline provision of a Traffic Service if radar coverage below MSA / terrain safe level is not considered adequate.
If you do not intend descending below MSA/terrain safe level, a De- confliction Service could be requested. The latter will be terminated / downgraded by ATC if you descend below MSA.