Connect a Grandstream desk phone directly to a NumberGroup SIP trunk

You don't need a PBX. For a sole trader, home office, or a small team where everyone has their own number, register the Grandstream desk phone directly to a NumberGroup SIP trunk and skip the middle layer entirely. Works on every Grandstream model we ship — GXP1625, GXP2135, GXP2170.

Members' Grandstream phones arrive pre-provisioned. Plug into any Ethernet port on your broadband router, plug in the power, and the phone auto-registers to your trunk — no web-UI config needed. The walkthrough below is only for phones bought elsewhere, or if you want to re-configure something specific (additional accounts, custom dial plan, dual-account setup).

About 5 minutes start to finish. Ask us to set the trunk's User-Agent Type to "RFC3261 Standard Trunk" and Send Numbers As to "United Kingdom"— that way the phone's Caller ID screen shows the familiar 01214001262 format, not +441214001262.

Step 1: log in to the phone's web UI

• Find the phone's IP address — press the round up arrow key on the handset, the screen shows the IP. On a GXP21xx, navigate to Menu → Status → Network.
• Browse to that IP from a computer on the same LAN: http://192.168.x.y
• Default login is usually admin / admin (older firmware) or a randomly-generated password printed on a sticker on the back of the phone (newer firmware). Change it immediately.

Step 2: configure Account 1

Go to Accounts → Account 1 → General Settings:

• Account Active: Yes
• Account Name: NumberGroup (anything memorable)
• SIP Server: sip.numbergroup.com
• Secondary SIP Server: leave blank
• Outbound Proxy: leave blank
• SIP User ID: T-AAAAAA-NNN from the email we send you (e.g. T-102377-001)
• Authenticate ID: same as SIP User ID
• Authenticate Password: your trunk password from the email we send you
• Name: your business or user name (shows on outbound Caller ID Name)
• Voice Mail UserID: *88 if you use NumberGroup voicemail, otherwise leave blank

Hit Save and Apply.

Step 3: SIP Settings tab

Go to Accounts → Account 1 → SIP Settings → Basic Settings:

• SIP Registration: Yes
• Unregister on Reboot: No
• Register Expiration: 60 (1 hour, default)
• SUBSCRIBE Expiration: 60
• Reregister before Expiration: 0
• Local SIP port: 5060 (default)

Under Codec Settings:

• Preferred Vocoder: PCMU, PCMA, G722 (HD voice), G729 — in that order
• SRTP Mode: leave as Disabled unless you've set the trunk Voice Mode to "secure encrypted voice"
• Symmetric RTP: Yes

Step 4: NAT handling

Under Accounts → Account 1 → SIP Settings → Advanced Features (or similar — naming varies by firmware):

• NAT Traversal: Keep-Alive
• Use Random Port: No

On the router side: disable SIP ALG. It rewrites SIP headers and almost always breaks something. Look for the setting under WAN / Firewall options.

Step 5: dial plan (optional but recommended)

Set a dial plan so the phone knows when you've finished typing a number and auto-dials. Under Accounts → Account 1 → Call Settings:

• Dial Plan: { [3469]xxxxxxxxxxx | [2-9]xxxxxxxxx | 1xxx | 0[1-9]xxxxxxxxx | 00xxxxxxxxxx. | 999 | 112 | 101 | 18000 | * | **x. }

That covers UK area codes, 0800, 03, 0844-style, 0044 international, emergency 999/112, NHS 111, and short codes.

Step 6 (optional): hotline for single-purpose phones

If the phone exists to call one number only (lift phone, reception emergency line, taxi rank), set the Off-hook Auto Dial under Call Settings to the destination. Lift handset → autodials.

Step 7: verify

• The phone's LCD should show Account 1: Ready within a few seconds
• We confirm the trunk has registered our side — ring us if you're not sure
• Dial out (ring your mobile) and confirm Caller ID
• Ring the DDI on your trunk from a mobile to test inbound

When it doesn't work

LCD shows "Account 1: Trying..." forever

• Wrong SIP User ID or password — re-check the email we sent with your trunk credentials
• Phone can't reach sip.numbergroup.com — check router and DNS
• Office firewall blocking UDP 5060 outbound

Outbound rejected with 403

Almost always means your Default CLI on the trunk isn't one of your owned DDIs. Ring us and we'll set Default CLI to a valid number.

Calls connect, no audio (one-way audio)

NAT issue. Confirm SIP ALG is off on your router and Symmetric RTP is enabled on the phone. If you have a strict firewall, you may need to allow UDP 10000–20000 outbound for RTP.

Set fraud caps before going live

Ask us to set Fraud Detection Simultaneous Calls to 2 (a single-handset trunk should never have 5 calls running) and a sensible Daily Spend. See our telecom fraud guide for why this matters.

Source: the device navigation in this guide follows Grandstream GXP21xx Administration Guide. We keep the click-path with the manufacturer so it stays current — the NumberGroup trunk values above are the part specific to your account.