Setup fail with 487 in good RF condition

Hello All.

If Invite is send at MO;
but MT registration going on;
and setup fail with 487 in good RF.

Can this be IMS issue?

Does the fail occur contionusly or only one time?

Sometimes randomly.

May be 2-3 percent of drive.

2-3 percent of short calls.

Too high.

Good to be check at IMS side, why registration takes too long and not completed on time.