National calls

The solution for national calls to landlines follows the same decision process as for local calls to landlines, although of course the numbers will be quite different. Whilst PSTN providers will tend to charge more for these calls, ITSPs are more likely to have a flat rate for local and national calls, or indeed bundle them into an attractive package. Therefore, you may find that routing via your ITSP becomes a more attractive option in this case. This should have the knock-on effect of reducing the number of PSTN lines required as a whole, saving online rental charges.

An area not yet considered is that of calls to mobile telephones (cell phones). ITSPs and PSTN providers have tended to exclude these calls from bundles, although as we can see from the earlier example, this situation is changing. However, for the best deals on calls to mobiles you frequently have to look at the mobile providers themselves. It is typical for a mobile provider to offer a large amount of included call-time for a relatively modest monthly fee, bringing the cost per minute down to 2 or 3 times the cost of a landline call.

With cost differentials like that, it frequently makes sense for customers to consider routing calls to mobiles directly onto the mobile network. For smaller companies, the means of achieving this is through the use of GSM gateways. Companies with high volumes of calls to mobiles have other options, such as a direct circuit to the mobile provider. There are many different types of GSM gateway, some designed to integrate with traditional analog PBXs, and others that are aimed solely at the VoIP age, presenting the GSM channels as SIP trunks. They also vary in size from those that will accept a single SIM card to rack-mounted devices that will take dozens or even hundreds.

For an even smaller implementation, it is possible to avoid the cost of a GSM gateway by use of the chan_mobile channel driver (see: http://www.chan-mobile.org/ for more information), which is a third party add-on to the standard Asterisk package. The channel driver allows the use of mobile phones as FXO devices, connecting to the Asterisk server through the use of Bluetooth. This driver also allows a Bluetooth headset to act as an FXS device. It is difficult to recommend this solution for use in a commercial environment, though, reliant as it is on the proximity of a mobile phone to the server and Bluetooth as the communication medium. This is especially so given that the one-off cost of a dedicated gateway device can be under $100.