CTI (Computer Telephony Integration) is one feature which is must for your PBX system. If your PBX can not support CTI, then most probably, you have bought an outdated and featureless EPABX system. No doubt, all PBX machines would integrate with CTI hardware with analog interfacing, but analog interfacing with CTI is almost dead and it will die soon, if not already dead.

Why Computer Telephony integration ?
Computer Telephony Integration can be defined as a technology which allows interaction between telephone system and a computer system. It extends features of telephone or voice communication to a PC software and vice-versa. There are many applications based on CTI, right from caller id display on a screen to more sophisticated and advanced IVR ( Interactive Voice Response) systems.
These CTI based applications help various office automation which result in increase in efficiency and reduction in cost of operation. There are many features, applications which can not be implemented on a PBX machine and those applications could be easily hosted on easily available PCs.
CTI feature in PBX systems encourage third party developers who develop various applications like IVRS, Voicemail, Call Center, Voice loggers etc. Third party developers can also customise many applications to enhance features of the PBX, as well as meet user requirements which may not be possible for a PBX manufacturer.

How to add CTI feature in PABX
Easiest way to provide CTI facility on a PBX is to support ISDN PRI protocol. PBX also has a RS232 port which can be used to send many call related information to a host PC for online billing system, automatic call distribution etc.
SS7 ( Signaling System Number 7 ) is another technology which can be used for many modern applications like SMS, media server etc. SS7 may be costlier or more difficult, but it offers many benefits over ISDN and can be used for building very large PBX or CTI applications.

Analog lines are also used for CTI applications like IVR, outbound IVR as they are cheap. But using analog lines for CTI application is on the decline for many disadvantages.

Few benefits to summarize of having CTI support in PBX systems :

1. Open support for CTI may develop developer community around your PBX machines who would develop many interesting and useful applications. Normally, third party developers create applications which may add real value addition to any PBX system.

2. CTI based application would create many unique applications which might be useful to be implemented on PBX system. So, CTI facility may help in PBX innovations. This also helps customization as per user requirements.

3. A large third party developers using CTI facility of a PBX machines may give advantage over competition as it helps popularise a PBX brand.