Evaluation of Traditional Direct Dial Service Options

ISDN PRI Circuit 

Big $$$ purchase ! Direct Dial service can be added to feature rich two way ISDN PRI circuits . Purchase is 23 channels at a time. A PRI with optional Direct Dial service will allocate any idle channel of it's 23 B channels  to handle an inbound DID call, an inbound main number call or an outbound call from any extension. This "multi-tasking" is called "Call by Call" service. Yet, 23 channels are more service than most companies need. Also, the naturally redundant POTS lines forming the original hunt group must be disconnected when the lead number is transferred to the one, non redundant ISDN PRI circuit. ISDN PRI is the only traditional choice if you desire Caller ID Name delivery.

T-1 Circuit

A simple cousin to ISDN PRI. Used for dedicated 800 service. Slower call setup. Also all or nothing service. You either have full 24 channel service or no service. Many phone customers have switched from analog service to a competitive carrier (CLEC) T-1 in order to get voice and data on the same pipe. While more advanced than the analog lines they replaced, T-1 circuits have this one limitation. They can only supply Caller ID number.

Analog Direct Dial Trunks (obsolete)

Expensive, few features. Unlike two way PRI channels, analog DID won't support Caller ID and only supports "inward" calls. So, one must order two sets of utilities that don't share traffic with each other. 

This split service architecture results in incomplete calls when either the last DID trunk or  the last POTS line is exhausted. Example: No one can make an outgoing call but your inbound DID trunks are idle!

Summary: Problems with traditional DID choices

All choices require $2000 - $4000 in added premise equipment.