IP Phone & Analog Line Parallel Connection!!

[WJ2600]

Hi Guys,

I need your ideas please,

Our Company needs to replace the old analog lines with an IP phones and get rid completely from the analog Mitel Peripheral cabinets,

The trick in this is that every user in the company has one phone in the office and the other one is in the room with same number,
both lines are analog and have the same number (parallel connection).

We are going to replace the office phones only with IP and keep the ones in the rooms Analog with same number,

We are still working on Rel. 5 SP1, because of the analog lines, and will keep using it after the replacement for the same reason,

I thought about the IP-Analog Adapters, the Ringing Group, and FXO/FXS gateways,

Any other simple idea to get this done??

Thanks,