In telecommunications jargon, trunking is the process by which several lines or connections are combined into one shared line. In the world of VoIP, SIP providers are increasingly offering products which provide the ability to make several calls simultaneously using a single account. Together with the possibility of being able to contact SIP participants via a shared switchboard number with individual extensions (DDIs), these types of accounts are also becoming attractive for business customers.
There are two possible options when using a SIP account with trunking:
- The customer retains his previous ISDN connection, along with any corresponding telephone numbers from the telephone company, and sets up an additional account having a separate number range with a SIP provider.
- The customer ports the numbers used thus far from the telephone company to the SIP provider and from then on uses the same numbers using SIP.
In this example we will take a look at a company planning to add a SIP trunking account, with up to 100 extension numbers, to its current ISDN point-to-point line having 20 extensions. The ISDN terminal devices with point-to-point line extensions used thus far can be retained. All new employees are to be issued with a SIP telephone with an extension via the SIP account.
Unique extensions are used since staff members have to be able to call each other internally. In order to migrate smoothly towards SIP, all ISDN terminal devices are to be contactable using both extension number and switchboard number of the SIP account. So an ISDN telephone should react in the same way for calls to '0123-12345 12' as it does for calls to '0456-54321 12'.
With the exception of emergency calls and service numbers, such as "0800" numbers, out-going calls are generally made using the SIP account. The signaling of SIP telephone numbers to call parties is paving the way for the medium-term discontinuation of ISDN telephone numbers.