Connect MicroSIP to a NumberGroup SIP trunk (Windows)

MicroSIP is the power-user's Windows softphone. Free, lightweight, no ads, no "Premium" upsell, full hotkey support, runs on a 256MB RAM laptop from 2010. Built on the pjsip stack so call quality is excellent. Windows-only, but if your sales team runs Windows it's genuinely the best free option.

When you ask us to set up the trunk, mention you want the User-Agent Type to "RFC3261 Standard Trunk" and Send Numbers As to "United Kingdom" — incoming numbers display in the familiar UK format.

Step 1: install MicroSIP

Download from microsip.org — the "Lite" build is 4 MB and uninstalls cleanly. Pick the Portable version if you want it on a USB stick / no Windows registry footprint.

Step 2: add the account

On first run, the Account dialog opens automatically. If you missed it, right-click the bottom-status bar → Add Account….

• Account name: NumberGroup
• SIP Server: sip.numbergroup.com
• SIP Proxy: leave blank
• Username: T-AAAAAA-NNN (e.g. T-102377-001)
• Domain: sip.numbergroup.com
• Login: same as Username
• Password: your trunk password from the email we send you
• Display Name: your business name
• Voicemail Number: *88 if you use NumberGroup voicemail, else blank
• Dial Plan: 9999 (or your office's outbound prefix; leave blank if you don't use one)
• Disable Session Timers: unchecked
• Allow IP Rewrite: checked
• Public Address (Rewrite): leave blank (let MicroSIP auto-detect)
• Register Refresh: 300
• Keep-Alive: 15
• Transport: UDP

Click Save. Within a few seconds the status bar at the bottom shows NumberGroup · Online.

Step 3: codec preferences

Right-click the bottom-status bar → Settings… → Audio Codecs:

• In Enabled column, top to bottom: PCMU, PCMA, G722, G729
• Disable everything else (drag to the Disabled column)

Under Audio:

• Echo Cancellation: on
• VAD (Voice Activity Detection): off — leave it as full audio, more reliable
• Output / Input device: select your headset specifically (not "default") — avoids audio jumping back to laptop speakers when you plug a headset in

Step 4: hotkeys (the killer feature)

MicroSIP's genuinely useful productivity feature: select a phone number anywhere in Windows (a web page, an email, your CRM), press a hotkey, MicroSIP dials it. Setup:

Settings → Shortcuts → Add:

• Modifier: Ctrl+Shift
• Key: C (or any letter)
• Action: Call selected text

Now Ctrl+Shift+C dials whatever's highlighted. Pair with the Windows clipboard manager and you've got 1-key click-to-call from every Windows application.

Step 5: network settings

Settings → Network:

• SIP Port: 0 (random) — works well on home networks
• RTP Port Range: 4000–5000 (or leave default)
• STUN Server: leave blank unless you're on a particularly strict corporate NAT
• Behind NAT: Auto-Detect

Step 6: verify

• Status bar shows NumberGroup · Online
• We confirm the trunk has registered our side
• Ring your mobile from MicroSIP — dial pad, top of window
• Ring your DDI from a mobile — MicroSIP should ring within 2 seconds

When it doesn't work

Status shows "Failed" or "Trying..."

• Right-click the status bar → SIP Log Messages for the raw SIP transactions
• Most common cause: wrong password — re-copy from the email we sent with your trunk credentials
• Second most common: corporate firewall blocking 5060

Calls connect but one-way audio

• Check Allow IP Rewrite is enabled in the account dialog
• Disable SIP ALG on the home router
• Windows Defender Firewall — allow MicroSIP through

Outbound rejected with 403

Default CLI on your trunk isn't a valid DDI. Set it in us.

Set fraud caps

Single-user softphone — set Fraud Detection Simultaneous Calls to 2 (ask us when you set up the trunk). See our telecom fraud guide.

Source: the device navigation in this guide follows MicroSIP — official site and documentation. We keep the click-path with the manufacturer so it stays current — the NumberGroup trunk values above are the part specific to your account.