Telephony terms often describe different layers of the same service. A number identifies a destination; a trunk connects systems; a contract assigns responsibilities. Use this glossary to keep those layers separate when reading an offer or writing a customer specification. It is a working reference, not a substitute for a provider’s technical documentation.
Numbers and connections
DID — Direct Inward Dialing
A number that allows an incoming caller to reach a business phone system without asking an operator to connect an extension. In hosted services, it can route to a queue, endpoint or other supported destination. It does not, by itself, specify simultaneous-call capacity. Start with numbers and SIP.
SIP — Session Initiation Protocol
A signaling protocol used to establish, modify and terminate sessions. SIP is not the audio stream itself. A working signaling exchange does not guarantee two-way audio, so test both. Protocol support alone also does not establish which features a particular provider has implemented.
SIP trunk
A service connection between a phone system and a provider using SIP. It can serve several numbers. The number of concurrent calls, permitted destinations, authentication method and failure behavior belong in the service specification rather than being inferred from the word “trunk.”
PBX — Private Branch Exchange
A business phone system that handles extensions and call routing. A PBX can be hosted or operated by the customer. Decide which party manages it and where the provider’s responsibility ends; otherwise a failed call can become a dispute over the support boundary.
Concurrent calls
Calls active at the same time. This is a capacity measure, not a count of phone numbers or employees. Ask whether a limit covers inbound calls, outbound calls or both, and what happens when the limit is reached.
Termination and forwarding
Termination delivers a call toward its destination. Forwarding takes a call arriving at one number and sends it onward to another supported destination. A forwarded telephone call may create an additional billable leg. Confirm the actual charging arrangement rather than assuming one call means one charge.
Number administration
Geographic number
A number associated with a defined numbering region. Its code can help a caller recognize a locality, but it does not prove where the person answering is physically located. Geographic eligibility and address evidence depend on the country, number category and applicable requirements.
Toll-free number
A number using a designated nongeographic code under a toll-free charging arrangement. The subscriber still pays for its service. International reachability and wireless-plan treatment need separate checks. The toll-free chapter covers the US administrative model and questions for a supplier.
RespOrg — Responsible Organization
The organization that reserves and administers US toll-free number records for a subscriber. A RespOrg may also provide telephone service, but administration and call carriage are different roles. Record who fills each role before accepting responsibility for a customer’s number.
Porting
Moving a number to another service provider under the applicable rules and process. Eligibility, authorization and account details matter. Do not cancel the existing service before an agreed transfer is complete and tested. A general statement that numbers are portable is not a promise of an instant change.
Business and compliance
White-label service
A service presented under the reseller’s branding while another company supplies some underlying functions. Branding does not decide legal responsibility or support scope. The agreement must explain provisioning, billing, customer records, abuse handling and what happens when the relationship ends.
Contribution margin
Revenue remaining after the direct costs included in a defined model, expressed as an amount or a share of revenue. It is not necessarily net profit. Always state which expenses are excluded. The pricing chapter uses an explicitly hypothetical example to show the calculation.
CDR — Call Detail Record
A record of call events used for billing and troubleshooting. Fields can include time, duration, destination and disposition, depending on the system. A CDR is not necessarily an audio recording. Keep sensitive call metadata access-controlled and apply an appropriate retention policy.
KYC — Know Your Customer
The process of establishing who the customer is and assessing relevant onboarding information. For numbers, this can involve identity, business and address evidence. Requirements are product-specific; see documents and KYC rather than assuming a single global document list.
STIR/SHAKEN
A framework for authenticating caller ID information on supported IP voice networks. It helps providers evaluate calling-number information; it does not establish that the conversation is safe or lawful. Operational implementation and regulatory obligations require separate review.
RMD — Robocall Mitigation Database
The FCC database for provider certifications and robocall mitigation information. An entry is not a general approval of a business or a guarantee that its traffic will be accepted. Read the US compliance chapter before deciding which entity must file and what its program must cover.