A DID is a public telephone number that can route an incoming call into a business phone system without requiring the caller to ask an operator for an extension. A SIP trunk is a connection arrangement that uses SIP signaling to connect a phone system with a service provider. A number and a trunk can be sold together, but they are not the same resource.
That distinction matters when a customer asks for “five numbers.” Five public numbers do not necessarily mean five simultaneous calls, five user accounts or five physical lines. The service needs a separate statement of call capacity and routing. Otherwise the customer and the supplier may agree on the quantity while imagining different products.
Start with the public number
A number is the address a caller dials. A DID service maps that address to a destination such as a PBX, a hosted extension, a call queue or another supported route. The destination can change without changing the number shown on the business website, subject to the service’s capabilities and terms.
The number’s category also matters. Geographic, national and toll-free numbers can have different eligibility, charging and reachability rules. Voice support does not imply text-message support. An inbound service does not automatically include an outbound calling account or emergency calling.
If you are evaluating an inbound virtual-number service, record the actual number type and supported termination methods. Treat the product page as a starting point for questions. It does not establish a reseller contract, white-label rights or permission to provision numbers in someone else’s name.
Then specify the connection
SIP is the Session Initiation Protocol. It is used to establish, modify and terminate sessions; it is not the audio itself. The IETF’s SIP specification, RFC 3261, is the primary technical reference for the protocol. Media transport and the specific service configuration are separate parts of a deployment.
A SIP trunk may carry calls for multiple numbers, while capacity is controlled separately through concurrent-call limits or another agreed model. Ask whether the quoted capacity applies to inbound calls, outbound calls or the combined total. Also ask what happens when the limit is reached: rejection, overflow routing or a billable burst are different outcomes.
Do not equate one SIP account with one person. Authentication credentials can connect a system, while extensions and individual user permissions are managed elsewhere. Keep trunk credentials restricted to administrators and store them outside public documentation or browser-side code.
Describe the service boundary
Write down which party owns each piece of the call path. A useful handoff document identifies the number provider, the trunk endpoint, the customer’s PBX, the internet connection, the answering devices and the person responsible for each configuration.
For a reseller, this is also a support agreement. If calls reach the customer’s PBX but the phone never rings, who investigates? If the customer’s internet is unavailable, is there a fallback route? If a codec or firewall setting prevents audio, which logs can be shared safely with the upstream supplier?
The customer does not need every implementation detail in a sales proposal. They do need a clear statement of the service, limits and support process. Keep the technical appendix for the people who operate the system.
Test signaling and audio separately
A call that rings is not necessarily a usable call. Test answer, two-way audio, keypad input where menus require it, hold, transfer and hangup. Include a longer call to detect a timeout that a quick test would miss. Use test accounts and agreed destinations, not emergency numbers.
Check the number displayed to the recipient and the information recorded in call detail records. These are useful for troubleshooting, but they should not be treated as proof of a caller’s identity. See the US compliance chapter for caller ID authentication and the limits of what it establishes.
The minimum product specification
Before quoting, complete this short specification:
- Number country, category and any required geographic locality.
- End-user identity and the party authorized to control or transfer the number.
- Incoming and outgoing capabilities, listed separately.
- Concurrent-call capacity and behavior when capacity is exhausted.
- Termination method, supported authentication and approved destinations.
- Renewal, usage, forwarding and support charges.
- Failure route, cancellation process and porting responsibilities.
Once these are explicit, the price comparison becomes meaningful. A low monthly number rental is not directly comparable with a package that includes call capacity, managed routing and support. Continue to reseller economics to separate those costs without confusing revenue with profit.