DID is the abbreviation for Direct Inward Dialing. This is a virtual local phone number for a given country, region, or city. Inbound calls to these numbers are forwarded as VoIP calls over Internet using a specific protocol (SIP, H.323, IAX) or are forwarded to a traditional telephone system, which includes both stationary and mobile phones.
A company usually uses to enable calls from clients in a given country without the need to have a physical presence (e.g. an office) there. Forwarding the calls via Internet allows significantly lower costs per volume of data traffic when compared to traditional international calls (whose costs are really prohibitive).
However, the primary reason DID numbers were invented was to enable the assignment of a direct number to individual employees without needing more physical phone lines (hence the term of “virtual numbers”). In fact, a single DID number can simultaneously process thousands of calls to an unrestricted number of extensions within a given company.
It is easy to imagine why this can be useful for a business. Having separate telephone lines for the CEO, commercial director, technical assistance experts, customer support, regular employees, or other types of employees can result in significant expenditure for establishing the infrastructure and high monthly costs for the services provided by the telephone company. DID numbers solve this problem.
Another important aspect is that of scalability. With physical telephone lines, one needs to make the decisions in advance with regard to how many lines are needs. This decision can later prove to be an over- or underestimation. Given that DID numbers are virtual numbers, their amount can be scaled up (or down) according to the needs almost instantly and with no modifications to the existing infrastructure.
There are several recommendations when looking out for a DID number provider:
To conclude, the particular needs each business has vary greatly as a function of the type, scale, and geography of the business, thus, besides the listed recommendation, many other specific ones might apply.