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Module 2
Technical Foundations
What you’ll get: how VoIP, SIP and routing work end-to-end, a practical equipment overview (GoIP, Skyline, Ejointech), and the differences between GSM gateways, SIM pools, LTE gateways, and eSIM—so you can pick the right stack.
Download Equipment Guide (PDF)
Watch Module 2 Video
VoIP & SIP — in one minute
SIP call lifecycle
What is VoIP?
Voice is digitized, split into IP packets, and sent over the internet instead of legacy TDM lines. Media (the actual audio) flows as RTP between the endpoints.
What is SIP?
SIP (Session Initiation Protocol) is the signaling that sets up, manages, and tears down a call. Think “who/where/how”; the voice itself travels via RTP. SIP can also handle features like hold, transfer, or adding participants.

INVITE
Your client/gateway asks to start a call; parties negotiate codecs and IPs/ports..
100 Trying
Provisional response from the remote side.
180/183
Ringing / early media (you may hear tones).
200 OK
The call is accepted.
ACK + RTP
Acknowledgement; two-way RTP media begins..
BYE
Either side ends the call; billing closes from CDRs.
Tip: keep NAT traversal in mind (public IP or proper forwarding), and prefer TLS/SRTP or IPsec where supported.
Routing basics
Routing chooses the path a call takes by prefix/country/operator with price–quality rules. Combine LCR (least-cost) with QBR (quality-based) thresholds for ASR/ACD/PDD and clean CLI. Add failover, retry timers, and CPS/CC limits; pick codecs (G.711/G.729/Opus) to balance bandwidth and voice quality.
  • Prefix & policy routing
    Choose a route by dialed prefix/country/operator. Apply rules: CLI requirement, max PDD, min ACD.
  • LCR vs QBR
    Least-Cost Routing minimizes price; Quality-Based Routing prioritizes ASR/ACD/MOS. Most stacks combine both with priorities.
  • Failover & redundancy
    Define primary/backup routes, timers, and retry logic. Keep CPS/CC (calls per second / concurrent calls) within limits.
  • Capacity & codecs
    Pick codecs (G.711, G.729, Opus) based on bandwidth/quality. Transcoding adds CPU and delay—avoid unless needed.
Equipment overview
A GSM/LTE gateway (GoIP, Skyline, Ejointech) hosts SIMs and converts SIP → mobile voice—your main voice engine. A SIM pool/SIM bank centralizes SIMs for remote assignment/rotation across gateways; LTE/VoLTE models add modern radio/IMS profiles. Pair with antennas, stable power (UPS), and an SBC/firewall at the SIP edge.
GoIP (Hybertone/DBL)
Well-known GSM/LTE VoIP gateways with 1/4/8/16/32/64 ports, web UI, SIP/H.323, SMS/USSD. SIM-bank support; easy to start; flexible rules.
Skyline
Carrier-grade gateways and controller software; strong remote management and monitoring options; good for multi-node setups.
Ejointech
Gateways and SIM bank lines (e.g., 32–128+ SIMs) with remote allocation/rotation; convenient for centralized SIM management.
Works with IP PBXs (Asterisk/3CX/etc.) and external control platforms (e.g., monitoring, anti-blocking, SIM lifecycle tools).
Profit & Unit Economics
Calculate margin and payback with a simple model. See which levers actually change results.
  • Core formula
    Gross Margin = (Payout/min − Effective Cost/min) × Billable Minutes.
    Everything in the business rolls up to these three variables.
  • Payout per minute
    The rate your buyer pays. Improve it by keeping clean CLI, good ASR/ACD, and reliable delivery; negotiate after a stable test period.
  • Effective cost per minute
    Your real cost to originate a minute: SIM plan, connectivity (ISP/VPN/IPsec), power/UPS, hardware depreciation (gateway/SIM server/antennas), and ops overhead (spares, space, maintenance).
  • Utilization & uptime
    More stable minutes/day = more margin. Control CPS/CC, prevent incidents, use monitoring/alerts and simple runbooks to reduce downtime.
  • Quality & disputes
    Buyers watch ASR, ACD, PDD, CLI, MOS. Keep test calls and CDRs; agree a lightweight SLA to avoid revenue loss in disputes.
  • Risk & compliance to include in the math
    Local regulation & SIM/KYC, SIM lifecycle/blocks, traffic-fraud patterns (short-stopping/refiling/CLI tampering), and power/ISP failures. Plan mitigations and budget for them.
Download sample P&L
Illustrative scenario for 12,000 daily minutes
(e.g., GOIP-8 at moderate load)
*for orientation only
Actual results depend on country, SIM/KYC, logistics, and how well you maintain QoS.
  • $0.035
    Payout /min
  • $0.018
    Effective cost /min
  • $204
    Gross/day: (0.035 − 0.018) × 12,000
  • $6,120
    Gross/month (×30)
Quick glossary
(for consistent wording across the course)
  • ASR: Answer-Seizure Ratio (% answered).
  • ACD: Average Call Duration (avg length of answered calls).
  • PDD: Post-Dial Delay (dial → ring/answer).
  • CLI: Caller ID presentation.
  • MOS: Mean Opinion Score (voice quality).
  • SIP trunk: IP connection for VoIP minutes.
  • Gateway: Device that originates calls into the mobile network using your SIMs.
  • CDR: Call Detail Records (billing/quality evidence).
  • BTS: Base Transceiver Station (cell tower).
How Module 1 works
From concepts to real-world choices—everything you need to see the full picture.
Step 1 — Business model
What GSM termination is, how you earn, and where margin comes from.
Step 2 — End-to-end flow
SIP trunk → Softswitch/SBC → GSM gateway → SIM/BTS → Callee.
What each piece does and how QoS is measured (ASR/ACD/PDD/CLI/MOS).
Step 3 — Market map
Signals & constraints in Africa, Asia, LatAm; principles to pick your first country.
Step 4 — Profit engine
Formula: (Payout − Cost) × Minutes. What drives results: quality, uptime, costs, buyer mix.
Go to Module 2: Equipment & Stack