hi,
if the ms is located in its home plmn, it has no reason to attach to another PLMN. The only scenario is if:
1/ the serving cell is not able to provide service to the MS (strong interfence in UL or in DL, cell is barred, low rxlev…)
AND
2/ there is no other cell from this PLMN around.
In this case, I think the MS could detach from the current PLMN and attach to the other PLMN. However, it will periodically attempt to re-attach to its home plmn.
cheers
pix