Mitel makes cordless devices that are compatible with the 5000 such as twinning a Mitel 5610 Wireless DECT phone, or an analog or SIP cordless phones can be used, twinning to cellular phone, ringing to overhead paging, and host of other possibilities. Remember that all the receptionist has to do is know the call is ringing, then she can simply pick it up from any phone and this can be accomplished with a variety of ways such as a common area ringer or flashing strobe light as examples.
As far as notifying others when she is away, this can be handled by some (maybe not so) simple programming, or Mitel Magic, as many of my fellow techs call it... Program the lines to ring directly to her phone, then depending on your setup, you can use call forwarding or night mode to send the calls to a an all ring hunt group when she is away from the desk or office. The system could also be programmed to say ring the main receptionist for XX seconds, then a group of phones for XX seconds, than all phones for XX seconds, then if all else fails, go to something else like a voicemail box or auto attendant as a last resort.
And everything dwayneg said is completely valid as well, there are literally half a dozen ways that this can be accomplished depending on your setup, licensing, office area, etc. and every one of them is correct and valid.
So this begs the question, are you an end user or a technician, and what is your comfort level with programming the system?
