Choose according to whether the assistant should answer first, take unanswered calls or operate only outside office hours. This guide supports discussion with the participating providers. It does not certify universal compatibility with a particular telephone system.
The diagram separates automated handling from a human route. Your actual plan also needs the public number, every intermediate destination and unanswered-call behaviour. Only then can costs and routing loops be assessed properly.
Key takeaways
The main points for using an AI phone assistant with an existing number are:
- The article distinguishes three connection options: forwarding, a phone-system integration and number porting; each changes a different part of the telephone service.
- With forwarding, the existing number remains with the existing telephone provider; porting is not automatically necessary for an AI trial.
- An AI minute allowance is not necessarily the whole telephone bill: the existing provider may charge for forwarding.
- A fallback must not depend entirely on the conversational logic of the failed component.
01 / 09
Forwarding, phone-system integration or porting: which option connects an existing number?
Forwarding. With forwarding, the existing number remains with the existing telephone provider. A rule sends calls to a destination number. This can be a straightforward starting point, but charges, caller ID, concurrency and return transfers still need checking.
Phone-system integration. A phone-system integration uses an explicitly supported connection to another service. Whether that is a SIP route, extension or another mechanism depends on both sides. General protocol support does not prove that the whole intended flow behaves correctly.
Porting. Porting moves the number to another provider. It is a separate telephone-service decision with contractual and migration consequences. It is not automatically necessary for an AI trial. Evaluate its business merits separately from the quality of a voice demonstration.
Forwarding sends calls to a destination number, a phone-system integration uses an explicitly supported connection to another service, and porting moves the number to another provider; the table compares what remains and what mainly needs verification.
| Option | What remains | Main verification |
|---|---|---|
| Forwarding | Existing number contract | Charges and forwarded-call behaviour |
| PBX connection | Depends on the agreed design | Supported connection and return transfer |
| Porting | Public number | New contract, timing and recovery plan |
02 / 09
What should you record about the existing call route?
Route elements. Record the main number, extensions, ring groups, queues, voicemail and schedules. Ask about less visible forwarding to mobile phones or cover arrangements. Those rules may continue acting after the assistant has made a new routing decision.
Rule ownership. Identify which application owns each rule. Business hours may exist at the carrier, in a PBX and inside the AI product. Conflicting schedules can make the assistant announce an open department while the phone system sends the call to voicemail.
Restoration. Document restoration as well. Who can disable forwarding? Which setting needs to be restored? Who then places an external test call? An unclear recovery procedure makes an otherwise reversible trial harder than necessary.
Written route. Do not rely exclusively on screenshots. A short written route showing what happens when each destination does not answer helps people understand the entire chain during an incident.
03 / 09
How do you set up call forwarding for the actual product?
IONOS's assistant availability instructions illustrate the use of an assigned destination number and forwarding of existing calls. Do not apply dial codes or menu instructions from one provider to another without verification.
Specify whether routing is immediate, busy-based, no-answer-based or scheduled. These may be independent rules. When AI answers after several rings, its greeting should account for the time the caller has already waited.
Check caller-number presentation and allow the caller to nominate another callback number. Incoming caller ID should not silently become a confirmed contact detail. A shared switchboard or withheld number makes that assumption especially unreliable.
04 / 09
Worked example: office overflow with a human alternative
Daytime overflow. An office wants staff to answer first during the day. If nobody answers, AI takes over and offers callback intake. If the caller asks for a person, it attempts a separate internal destination. That destination must not forward unanswered calls back to the public number.
Busy line and after hours. If the internal line is busy, an owned callback task is created. After closing, another message describes the next genuinely staffed period. The assistant does not promise an immediate response unless an appropriate service has been organised.
AI outage test. During acceptance, deliberately make the AI unavailable. Telephony should follow the agreed independent alternative. A fallback must not depend entirely on the conversational logic of the failed component; an instruction in the prompt cannot restore an unreachable service.
After recovery. Repeat the test after restoring normal operation. Recovery should return to the intended production route, not leave a temporary test rule active. Record the final configuration so later staff know which state was approved.
Got a workflow like this?
Tell me about it. Honest take and a rough price, usually within a day.
05 / 09
What does the whole call route cost?
More than the AI bundle. An AI minute allowance is not necessarily the whole telephone bill. The existing provider may charge for forwarding. Transferring to a person can add connection segments. Confirm the applicable items in the real tariffs.
Synthetic calculation structure: total monthly cost consists of existing telephony, the AI base charge, billable AI usage, additional connections and support. Do not assume zero for an item before confirming it. The time billed for a forwarded connection may differ from the time during which the AI is speaking.
Concurrency. Check simultaneous-call capacity as well. Sufficient monthly minutes do not show whether three callers can be served at the same time. Ask to see the behaviour when capacity is exhausted and size the arrangement for your actual peaks.
Who bills each segment. A useful supplier quote identifies who bills every segment. This makes it possible to compare two routes without accidentally counting the same charge twice or omitting the connection between services.
06 / 09
What differs between Austria, Germany and Switzerland?
Numbering rules. The number must suit the country and intended service arrangement. Austria's RTR explains geographic telephone numbers, while BAKOM describes Swiss telephone numbering. For Germany, confirm provision and porting details with the provider responsible for the German service.
Foreign destination numbers. A foreign destination number can introduce different charges or contractual conditions. Ask explicitly about destination country, reachability from customer networks and number display on return calls. A German advertised product price is not automatically a complete calculation for an Austrian or Swiss route.
Several locations. For several locations, the assistant should know which public number was dialled. That can determine language, opening hours and receiving team. Check whether the chosen connection preserves this information or whether another reliable mapping is required.
07 / 09
Which tests should the connection pass?
Test these cases when connecting an AI assistant to an existing number:
- calls from fixed and mobile networks
- withheld caller ID
- busy and no answer
- opening-hours rules
- at least two simultaneous calls
- return transfer to a human
- complete AI-service failure
Inspect the resulting work item as well as the conversation.
For each test, record the originating route, destination, schedule, expected outcome and deviation. Avoid unnecessary real customer information. Both the telephone administrator and the team processing callbacks should assess the result.
A commissioning decision needs a concrete call-flow plan, cost assumptions and named responsibilities. Those details make it possible to keep the existing number while understanding exactly what the new answering service changes.
08 / 09
Take the worksheet
Worksheet
Download the editable CSV table
| Question | Required decision | Test evidence |
|---|---|---|
| When does forwarding activate? | Immediate, delayed, busy or scheduled | Call each configured state |
| Which caller number is visible? | Original caller or forwarding number | Mobile, landline and withheld calls |
| Which call legs are charged? | Incoming and forwarded segments | Identified test call on billing record |
| Where does human transfer go? | Destination bypassing AI | Available and busy staff |
| How is service restored? | Named owner and rollback instructions | Deliberately unavailable assistant |
Columns to fill in, in the file: Your value / observation, Owner, Next action.
Download the editable CSV table (CSV). Add your own values, owners and open actions. This worksheet contains no measured client results.
09 / 09
Related decisions
These pages cover the service overview and the related decisions for keeping an existing number with an AI phone assistant:
- AI phone assistants: functions and boundaries
- Plan an AI phone assistant integration with 3CX
- AI phone assistants and Microsoft Teams call routing
- Human handover and callbacks for AI phone assistants
For a workflow review, start with the task, systems involved and typical exceptions. Discuss it with modernagents.
