SIP Trunk Provisioning
Explained for Business Owners

SIP trunk provisioning is the process of setting up internet-based phone lines for your business. In simple terms, your SIP provider creates a voice account for your business on their network (their side of provisioning), and you enter the login details they give you into your business phone system (your side of provisioning). Once both sides are configured, your phone system can make and receive calls through the internet instead of through old-fashioned telephone wires.

Think of it like setting up a new email account: your email provider creates the account (provisioning on their side), you enter your email address and password into your email app (setup on your side), and then you can send and receive emails. SIP trunk provisioning works the same way — but for phone calls instead of emails. The result is the same business phone number, the same PBX system, the same handsets your team already uses — but with a lower monthly bill and more flexibility.

In most cases, no — you do not need new phones or hardware to start using SIP trunking. If your business already has a PBX (a phone system with extensions), it almost certainly already supports SIP trunking. SIP has been the industry standard for business phone systems since the early 2000s — any PBX made in the last 15 years (FreePBX, Asterisk, 3CX, Cisco, Avaya, Mitel, Grandstream, Yeastar, and many others) can connect to SIP trunks without hardware changes.

Your existing IP desk phones (the phones your staff use) also do not need to change — they connect to your PBX system as before, and the PBX handles the SIP trunk connection to the carrier. The only exception is if your current phone system is very old (pre-2005) and uses only analogue or digital ISDN handsets rather than SIP — in that case, a small adaptor device (an Analogue Telephone Adaptor or ATA) can bridge the gap without requiring new handsets. NetViaVoice will advise on the best approach for your specific setup during the free initial consultation.

Yes — you can almost always keep your existing phone numbers when switching to SIP trunking. The process of moving a phone number from one carrier to another is called "number porting," and it is a legal right in most countries (the US, UK, EU, Australia, Canada, and many others have number portability regulations that require carriers to release your numbers to a new provider).

The porting process typically takes 5–10 business days for US numbers, 7–15 business days for UK numbers, and 2–4 weeks for some international numbers. During the porting period, your number continues to work on your old line — there is no interruption to incoming calls. On the day porting completes, your number automatically starts receiving calls on your new SIP trunk, and your old line can be cancelled.

NetViaVoice manages the porting process on your behalf — you complete one simple authorisation form (called a Letter of Authorisation or LOA) and we handle all communication with your existing carrier. There is no risk of losing your number during a properly managed porting process.

The number of SIP channels you need equals your maximum number of simultaneous calls during your busiest hour — not your total number of staff or extensions. Key benchmarks for small businesses:

• 5–15 person team (light phone use): 3–5 channels typically sufficient
• 15–30 person team (moderate phone use): 5–10 channels
• 30–50 person team (heavy phone use): 10–18 channels
• Sales teams or customer service: Multiply active agents by 0.8–0.9 for typical concurrent call rates

The practical approach: look at your existing phone bill and find how many lines or circuits you are currently paying for. Identify the busiest hour of your day and estimate how many calls are actually in progress at once. Most small businesses have 20–30% of staff on calls simultaneously at peak times — that is your channel count. Always add 2–3 channels as a buffer.

The good news: with SIP trunking, getting the number wrong has no penalty. You can add channels within minutes if you have too few, or reduce channels to save money if you over-estimated. Unlike PRI lines (which you pay for in 23-channel blocks regardless of usage), SIP channels are individually adjustable at any time.

This is the most common concern business owners raise about SIP trunking — and it is a valid question with a reassuring answer when you set things up correctly.

If your internet connection fails, your SIP trunk will also stop working while the internet is down. However, this risk is manageable through two complementary protections:

• Carrier-level failover: NetViaVoice monitors whether your SIP system is reachable. If it detects your system has gone offline, it automatically forwards all incoming calls to a mobile number or alternative number you designate — within 60–90 seconds of the outage starting. Your callers reach someone; they just don't know the internet went down.
• Secondary internet connection: Many businesses add a 4G/LTE mobile router as a backup internet connection. If the primary broadband fails, the router automatically switches the SIP trunk traffic to 4G — completely transparent to callers and staff.

In practice, this dual-layer protection means most businesses using SIP trunking properly experience less downtime than they had on traditional phone lines — because SIP failover activates in seconds, while a failed PRI circuit typically requires a 4–8 hour engineer visit to repair.

For the full redundancy architecture, read: SIP Trunk Provisioning for Enterprises.