Extension count is not the number that matters — concurrent calls is. This page shows you how to size a PBX properly, what a complete system includes, and how to move to IP without losing the landline number on your signboard.
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Indicative only. Site conditions change the final design — we confirm on survey.
The headline product is rarely more than half the installed cost. These are the parts that get left out of budgets.
The call engine. Sized on concurrent calls, not extension count.
One per desk. Entry models for general staff, multi-line for reception.
Powers the phones over the data cable. Removes a power adapter per desk.
SIP trunk for internet calling; FXO to keep your existing landline.
Cat6 to every desk. Voice and data share the drop with a PoE phone.
Keeps analog devices alive — fax machines, lift phones, older intercoms.
Routes calls to mobile numbers over a SIM. Usually the largest saving for a Bangladeshi SME.
TF card, USB or network storage. Required for banks, insurance and call centres.
Because the phones draw from the switch, one UPS keeps the whole phone system alive.
The honest trade-offs.
| Consideration | SIP Trunk | FXO (landline) | GSM/LTE Gateway | Hybrid |
|---|---|---|---|---|
| Keeps your published number | Depends on porting | Yes — unchanged | No | Yes |
| Cost to mobile numbers | Moderate | Highest | Lowest | Lowest |
| Cost to landlines | Lowest | Moderate | High | Lowest |
| International calling | Cheapest | Expensive | Expensive | Cheapest |
| Works without internet | No | Yes | Yes | Partly |
| Setup complexity | Low | Low | Low | Moderate |
| Best for | New offices, remote teams | Keeping an established number | Heavy mobile calling | Most Bangladeshi SMEs |
Network quality first. Voice is far less tolerant than data. Packet loss above 1% is audible. If your network was built for browsing, test it before ordering phones.
VLAN separation. Voice on its own VLAN with QoS. Sharing a flat network with file transfers and CCTV is the most common cause of choppy calls.
PoE budget. Add up the wattage of every phone plus 20%. A switch that runs out of PoE budget drops phones silently and intermittently.
Bandwidth per call. Roughly 100 kbps per concurrent call on G.711. For 9 concurrent calls that is about 900 kbps of guaranteed headroom, not shared with everything else.
Existing line survey. Before quoting, confirm how many landlines you actually have and which are live. Most offices are paying for lines nobody uses.
Power for the switch. With PoE phones, the switch is the single point of failure. One UPS there protects every phone in the building.
Call charges are where the saving is. Internal calls between extensions cost nothing, including between branches. For a company with seven branches, inter-branch calling drops to zero on day one.
GSM gateway savings. Most Bangladeshi office calls go to mobile numbers. Routing them over a SIM inside the PBX rather than through a landline carrier is typically the single largest reduction in monthly telecom cost.
No per-seat licensing on most SME platforms. Asterisk-based systems like OpenVox and ZYCOO include the feature set. Confirm this before choosing a platform — some vendors licence per extension and the cost compounds.
Phone lifespan. IP phones are simple devices and typically last 5–7 years. The handset cord and hookswitch fail before the electronics do.
The cost people forget: the old PABX maintenance contract you keep paying after migration. Cancel it explicitly — it does not lapse on its own.
Every one has cost a real project real money.
A 60-person office buying a 60-call PBX pays for six times the capacity it will use. Concurrency in a general office is 10–15%.
Do this: Size on concurrent calls. Use the calculator, then add 30% headroom for growth.
A full migration in one night means every problem surfaces at once, on Monday morning, with no fallback.
Do this: Run old and new in parallel. Move one department, prove it, then move the next.
This is the fear that stalls most migrations — and it is unfounded. FXO ports bring your existing landline into the PBX unchanged.
Do this: Keep the landline on FXO. The number on your signboard keeps ringing.
Calls sound fine until someone uploads a large file. Then they break up, intermittently, and it takes days to diagnose.
Do this: Separate VLAN with QoS from day one. Retrofitting this is far harder.
Fax machines, lift emergency phones and older intercoms do not speak SIP. Discovering this after cutover is a bad day.
Do this: Audit every analog device first. FXS ports keep them all working.
Phones drop off the network at random and nobody connects it to the switch budget. It looks like a network fault for weeks.
Do this: Total the phone wattage, add 20%, then choose the switch.
The PBX is the easy part. Extensions, concurrent calls and how you connect to the outside world are what decide the design.
Size the PBX on concurrent calls, not extensions. Then choose phones by who uses them.
Concurrent call capacity is the real limit. Ask for it in writing; extension count is the marketing number.
Every product above is one DigiMark Solution imports, stocks and warrants. Pricing is quotation-based and depends on quantity and configuration — request a quotation or ask on WhatsApp.
What an IP telephony tender must state to produce comparable quotations.
| Sector | What to specify | Typical scale | The clause bidders exploit |
|---|---|---|---|
| Corporate office | Extension count AND concurrent calls; SIP trunk channels; voicemail; call reporting | 50–500 extensions | Concurrent calls, stated separately from extensions |
| Hospital | Ward and department dialling plan; emergency call handling; redundancy; 24/7 operation | 200–1,000 extensions | Specify what happens to an in-progress call during a failover |
| Hotel | Room extension billing; wake-up calls; front-desk integration; housekeeping status | Per room | PMS integration must be named and tested, not assumed |
| Call centre / support desk | Queueing, recording and agent reporting; concurrent call capacity at peak | 20–200 agents | Peak concurrent calls, and the recording retention in months |
| Multi-branch | Per-site survivability on WAN loss; central dial plan; inter-branch calling | Varies | Local survivability is rarely quoted unless it is specified |
| Government | Recording retention; audit logging; on-premises only where required | Varies | State whether cloud hosting is permitted — it usually is not |
Multiply your extension count by your busy-hour concurrency. A general office runs 10–15%, sales teams 25–35%, call centres 70–90%. A 60-person office typically needs 9 concurrent calls, not 60. Sizing on extension count is how companies overspend by four to six times. Use the calculator above.
Yes. FXO ports bring your current PSTN lines into the IP-PBX unchanged, so the number printed on your signboard, business cards and invoices keeps ringing exactly as before. This is the single most common concern that stalls IP migration, and it is entirely solvable.
Yes, with FXS ports. Fax machines, lift emergency phones and older intercoms do not speak SIP, but FXS ports keep them connected to the new system. Audit every analog device in the building before migration — this is where surprises happen.
The largest saving for most Bangladeshi companies is a GSM/LTE gateway, which routes calls to mobile numbers over a SIM inside the PBX rather than through a landline carrier's inter-network tariff. Inter-branch calling also drops to zero — for a seven-branch company that alone is significant.
Roughly 100 kbps per concurrent call on the G.711 codec. Nine concurrent calls needs about 900 kbps of guaranteed headroom — not shared with file transfers and CCTV. Voice is far less tolerant of packet loss than data, so network quality matters more than raw speed.
No. Run the old PABX and the new IP-PBX in parallel, move one department, confirm it works for a week, then move the next. A single-weekend cutover means every problem surfaces simultaneously on Monday morning with no fallback. Staged migration is why FXO matters — it lets both systems coexist.
Free site survey across Dhaka and Chattogram. Our engineers confirm the design before anything is quoted.