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How Many SIP Trunks Do I Need?

Stream - The digital backbone to your business

With the upcoming closure of the UK’s PSTN network by 2025, businesses across the country are facing a critical decision on what to do with their business phone lines and Telecom systems. Migration to SIP trunking using an internet connection is a simple and cost-effective method but how do you work out how many SIP trunks you need to support your business?

What is a SIP Trunk?

SIP stands for Session Initiation Protocol and is the standard and widely used communications protocol for voice and video traffic in a Unified Communications system on an Internet Protocol data network; see our blog on how SIP Trunking works for further details.

Sizing the number of SIP Trunks for your PBX system

Legacy PBX phone systems connect to the public switched telephone network (PSTN) using a fixed amount of ISDN primary rate interface (PRI) circuits or analogue telephone lines, which can be costly and time intensive to change or modify.  A SIP trunk channel can be compared to a single ISDN channel or analogue phone line. Therefore, one SIP trunk channel is required for every simultaneous or concurrent call generated by the IP-PBX or SIP gateway.

There is technically no minimum or maximum limit to the amount of SIP channels that make up a SIP trunk and adding additional SIP channels is quick and easy for your SIP trunk provider to arrange, some providers allow customer self-management through a portal.

Unlike legacy ISDN or analogue solutions, the number of channels a SIP trunk provides is flexible and easily sized to suit business communications needs.  A rough guide is that for a larger company a traditional SIP trunk ratio of three employees to one SIP channel (3:1), this requirement will vary depending on usage and call centres may need a higher ratio, depending on call volume and business phone telecommunication requirements.  VoIP calculators are available to size a SIP trunk from your service provider and determine the number of concurrent calls and call paths required for your communication system.

How much bandwidth does a SIP Trunk need?

G.711 and G.729 are two common codecs to consider when sizing SIP trunking requirements. G.711 provides high-quality voice calls and will use around 87.2Kbps of bandwidth per call, this is broadly representative of a 64Kbps ISDN channel or analogue line.  G.729 uses compression algorithms to reduce total bandwidth and each SIP trunk channel will use a total of 31.2Kbps per call.  Another simple formula is roughly 1Mbps can handle 10 G.711 simultaneous calls.  There are also other codecs that use different sampling rates and sizes depending on requirement, i.e. G.722, G.723.

Switching to our SIP trunk service can save your business £1,000’s

From £5.00 per SIP trunk per month 

Can I port my existing phone numbers to SIP?

Yes any existing UK landline number can be migrated to a SIP Trunking Service.  This doesn’t affect the number of SIP Trunks you need as you could have 100 numbers allocated with a single SIP trunk however if you do have 100 inbound numbers you may need more than a single SIP trunk to handle the concurrent calls!

How much do you pay for calls with SIP Trunking?

Typically SIP Trunks now include inclusive calls within the cost of the SIP trunk.  Stream Networks provide 5000 minutes per month to UK local and national numbers, and 2000 minutes to UK mobile phones per month when signing a 3 year contract.  We also provide bespoke call routing and prices for International calls.  View our dedicated SIP Trunking page for further information.

The benefits of SIP Trunking

The benefits of SIP trunking are flexibility and scalability over traditional telephony services. A VoIP solution is an effective method of future-proofing business solutions and sip trunk pricing can deliver cost savings over traditional phone lines both online rental and call costs.

If you are a small business then you may want to also consider moving to a hosted telephony solution where the PBX is hosted in the cloud, with SIP Trunks and Handsets included in the monthly rental price as this will save on installation and maintenance of a dedicated PBX for your business.

As with any business telecoms solution always check your business needs to ensure you get the best solution and phone calls for your business.

If you would like further information on Stream Networks business telephony solutions please complete the contact form below or contact us by telephone on 01635 884170 or by email to sales@stream-networks.co.uk


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