What Is A Mobile Gateway?
A mobile phone gateway is basically a mobile phone (so it still needs a SIM card) in a small box which connects to an existing PBX system via a cable or, in the case of a SIP mobile gateway, via Ethernet. They come in GSM and WCDMA variants including what Telstra classes as Next G. The dual band GSM units seem to be the cheapest and with the exception of data, will provide everything you need.
SIP Mobile Gateways
A SIP mobile gateway (one such product is the Voice Blue units) allows you to connect the gateway directly into your local area network and have calls from your VoIP PBX system pass through to the mobile network instead of a VoIP provider or landline service. One distinct advantage of having a SIP mobile gateway is that the server will generally be able to tell what state the call is in (ringing, busy, answered) and will reflect this in the call detail records and allows you to also have a mobile phone number as part of a ring group.
Can I Connect An ATA (Analogue Telephony Adapter) And An Analogue GSM Gateway?
This is generally cheaper, but has a couple of down sides.
- No Call Control Signalling. Once the call is handed off to an analogue device, it can no longer signal a VoIP system (such as Asterisk) that the call has been answered, is still ringing or that the line is busy. This is a major problem if you have a large office and people divert their desk phones to a mobile, and becomes an even bigger problem if their phone is part of a ring group. (If your phone is part of a ring group and a phone call comes in, the phone call from the ring group generally follows the divert you have set on your local IP phone meaning the external caller is now only ringing on your mobile phone.)
- Diminished Audio Quality. I have heard reports from people who use analogue mobile phone gateways that the audio quality is not that great. Compare this with a SIP mobile gateway which provides really good audio quality.
Advantages Of A Mobile Gateway
Assuming that you are a company that has a fleet deal with a carrier (i.e. all your company mobiles are with one mobile phone company) and you get free calls between your mobiles, you can set your intra-company calls and diverted calls to go via the gateway and have the benefits of either free calls or reduced tariffs depending on the plan you have with your carrier. A mobile gateway may also be cheaper for day to day calling as many landline providers charge high rates for mobile telephone calls. For example, some plans charge $0.30 / 30 seconds when some mobile plans charge $0.12 / minute.