Thank you for your replies, I configured a Trusted device and set the First Alternative (or Always Alternative) rerouting to a speed call and it looks it does what I want :-).
VinceWhirlwind, I did not get the idea. How can the mobile phone be a device in an MDUG? A speed call cannot be added as a member... "A member must be assigned to a single line, multiline, or analog set or be a hotdesk user.". I think this can be done via EHDU but it will eat an EHDU license.