Calls fall back to reception when using Mobile Extension

[TJ79]

Hi,

When a Mob.Extension user calls a number this call will be forced via the Mobile operator to the PBX with the R1 nr prefixed so that the call will be handled.
When a Mob.Extenion user missed a call and calls back to this number it sometimes happens that the call ends up at the reception with the message of the reception that he's calling himself.
Any idea how this could happen?
Somehow my guess is that the mobile extension number of this user that wants to make te call back i somehow busy in the system and when this is the case it tries to setup the call via the forced on pabx functionality and enters for the 2nd time the R1 number which is already busy.
But how to solve this as the user isn't busy?

Thanks in advance for your reply