Call transfers for AI phone assistants: from cold transfers to warm transfers

Summary

AI phone assistants can automate many processes, but there are still situations where calls must be handed over to human employees. A cold transfer forwards the call directly, while a warm transfer enables a controlled handover where the employee is informed in advance about the customer's issue. Technically, such transfers are typically implemented via SIP REFER for simple transfers or SIP INVITE for controlled and interactive handovers. VoiceBooker supports both approaches and provides special stages for warm transfers for automatic call summarization as well as for handling failed transfers.

Introduction

Although AI phone assistants can now automate many business processes, there are still situations where a human needs to take over. Complex customer issues, sensitive decisions, individual advice, or escalations cannot always be fully automated. For an efficient handover, it is helpful if the receiving person already knows what the conversation is about. Similar to a secretary who briefly summarizes a call before passing it on to management, an AI phone assistant can ensure that the receiving person is ideally prepared.

Why call transfers remain important

The goal of a voice AI system is not necessarily to handle every call completely on its own. Often the greatest value lies in automating routine tasks, pre-qualifying information, and routing customer issues efficiently to the right place.

Typical scenarios for a transfer include:

• Escalations from dissatisfied customers
• Complex technical issues
• Sales inquiries with high revenue potential
• Legal or contractual questions
• Follow-up questions that require human decisions

In all these cases, the quality of the handover significantly affects the customer experience.

Cold transfer vs. warm transfer

In classic telephony, there are two basic types of call transfer.

Cold transfer

In a cold transfer, the call is forwarded immediately to another person or department. The original interlocutor is connected directly, without informing the target person in advance. The process typically looks like this:

1. The customer speaks with the assistant.
2. The assistant recognizes that a transfer is required.
3. The call is forwarded directly to the target phone number.
4. The target person answers the call.

The advantage of this method is its simplicity and speed.

The disadvantage: the receiving person knows neither the conversation history nor the caller's specific issue and often has to ask the same questions again.

Warm transfer

A warm transfer is more controlled. Before the customer is connected to the target person, that person is first contacted and informed about the issue. The process might look like this:

1. The customer describes the issue.
2. The AI assistant creates a summary.
3. The target person is called.
4. The bot provides a summary of the conversation.
5. The target person decides whether to take over the call.
6. Only then is the customer connected.

This process is similar to the classic secretary model and ensures much more efficient conversations.

Advantages of a warm transfer:

• No need for the customer to explain everything again
• Faster problem resolution
• Better preparation for the employee
• Higher customer satisfaction
• Possibility to accept or decline the call

Technical implementation in SIP

On a technical level, call transfers are typically implemented via the Session Initiation Protocol (SIP). Two basic methods have become established.

SIP REFER

A cold transfer is often performed using the SIP method REFER.

The current telephony system tells the endpoint:

Connect directly to this other party from now on.

After the REFER, the target system takes over the call completely. The original system leaves the signaling chain.

Advantages:

• Very efficient
• Low signaling load
• Simple implementation

Disadvantages:

• No control over the further flow
• No possibility of advance notice
• No acceptance or rejection logic
• The call is handed back to the carrier, so if the destination does not answer or is unavailable, the conversation cannot continue inside the bot

SIP INVITE

Warm transfers are often implemented with additional SIP INVITE connections. The original system remains active at first and creates a second call to the target person itself.

This creates a controlled handover process:

1. The customer stays in the queue.
2. The system calls the target person.
3. The target person is informed.
4. The target person can accept or decline the call.
5. Only afterward are both parties connected.

Because the system retains control over both call legs, additional logic can be implemented.

If the destination does not answer or is unavailable, the original bot flow can continue. That means the assistant can keep the conversation going, for example by collecting more details about the inquiry, offering a callback, or routing to an alternative contact.

Examples:

• Reading out a call summary
• Passing on important customer data
• Prioritizing certain calls
• Choosing alternative contacts
• Handling busy or timeout situations

For modern AI phone assistants, this approach is especially attractive because it goes far beyond a simple transfer.

Warm transfers with AI

The real strength of voice AI is that the AI already collects important information during the conversation.

This can include, for example:

• Caller name
• Customer number
• Product or contract
• Customer issue
• Previous conversation results
• Urgency of the request

Before a transfer, the AI can automatically create a concise summary from this information.

Instead of receiving an unprepared call, the employee might get something like:

Mr. Miller is calling about a contract extension. He is interested in the premium package and has already asked questions about pricing. The conversation was escalated to sales because of an individual discount request.

The employee can then enter the conversation in a much more targeted way.

Call transfers in VoiceBooker

VoiceBooker supports both Cold Transfers and Warm Transfers and therefore enables flexible integration into existing business processes.

For cold transfers, the call can be forwarded directly to a target phone number.

For warm transfers, VoiceBooker provides an extended flow based on additional stages.

Stage 1: transfer assistant

At the start of the warm transfer, a new stage is activated.

This stage receives a special prompt with the task to:

• Summarize the previous conversation
• Explain the customer's issue
• Inform the target person about the call

In addition, actions or functions are available that allow the target person to:

• Accept
• Decline
• Forward to an alternative contact

This creates a controlled handover process with full transparency.

Stage 2: failed transfer

Not every employee is available.

It can happen that:

• The line is busy
• Nobody answers
• The call is rejected
• Technical problems occur

For this case, VoiceBooker automatically activates a second stage.

This stage uses a short prompt intended exclusively for failed transfers.

Possible tasks for this stage include:

• Inform the customer about the failed transfer
• Look for an alternative contact
• Offer a callback
• Record a message
• End the conversation in an orderly way

This keeps the conversation flow intact even when the actual transfer is not successful.

Conclusion

Even in a world of powerful AI phone assistants, call transfers remain an important part of many business processes. While cold transfers enable fast and simple forwarding, warm transfers offer much more control and a significantly better customer experience. SIP-based controlled transfers can inform the target person about the issue before the conversation starts, and that person can decide whether to take over the call. With VoiceBooker's integrated transfer functions, both simple forwarding and intelligent warm transfer scenarios with automatic call summarization and error handling can be implemented efficiently.