The phones that I have are Norstar M7310 desksets, that came with a Norstar system. For a picture, see beagle-ears.com/lars/enginee … /m7310.gif
These phones have been standard for
- Norstar (Nortel’s small business hybrid key system)
- Meridian (Nortel’s medium to large business PBX system)
- and Nortel’s DMS-100 based public ISDN CENTREX service.
They were produced for over 20 years.
** U interfaces versus S0 interfaces **
The U interface is a two-wire interface designed to run for long distances (central office to customer premises) over a single pair, providing power to the customer premises equipment over that pair. Only a single device can be at the customer end of that pair; the intent is for that to be a network termination unit (NTU) that provides an S/T bus on the customer side.
The S interface is a bus allowing multiple pieces of equipment (up to 8) to share services and transfer calls between each other on a TDM bus providing two B channels (each representing a voice call) and a D channel for signalling.
In North America, the regulatory ruling said everything on the customer’s premises must be owned by the customer so that there would be a competitive market for NTUs. In Europe, the regulations said that the telco must deliver a standard interface to the customer, so that the telco should install the NTU in the customer’s building.
In an environment where the customer had to buy the NTU, it was simpler to integrate the NTU into the user equipment than to separate the functions into multiple boxes.
Since Nortel was already building the ISDN telephones connecting to the
U interface, they decided to use it for the inside wiring in their PBX and hybrid systems. This led to the anomaly where my Norstar system uses ISDN-BRI on the station side, even though no ISDN trunk cards are available for it. (Newer Norstar systems to offer BRI trunks).
** M7310 as generic desksets **
The M73xx telephones use “stimulus signalling”; i.e. when you hit a key on the telephone, the phone sends “user hit key no 17”. All call handling is performed by the switch and all keys are programmable (except RLS ?)
Because these phones are so common and their design so close to industry standards, many, many VoIP switches support them.
And because they are almost as common in the US and Canada as the WE-2500 (and as well built), you can find them everywhere.