ISDN lines are the norm for external connections (primary rate BTISDN30 or BT branded ISDN2 for smaller businesses). These are not the same as high-speed broadband data transfer, which is not suitable for voice calls unless you are planning to embrace the latest voice over Internet protocol (VoIP) technology.
ISDN lines also offer important extra services such as Direct Dialling Inwards (DDI), Caller ID and External Transfer.
One of the most important parts of external connection planning involves the accurate estimation of how many lines (trunks) you require. In particular, your assessment of likely call volumes will indicate the ration of trunks to extensions that you will require to deliver the required service. Too high a ratio of extensions to trunks will mean that your staff struggle to get an outside line 0 while inbound callers find that your lines are always busy. This is where phone system planning can really have an impact on your day to day business.
When you need more help with your system configuration, Telephone Systems (UK) Ltd can help you select a knowledgeable phone systems dealer near you.