Hi, I want to allow the assistant to transfer a call to our call center, but I'd like to be able to tell the call center what the customer's ID is so once the call is connected to the agent they know who the customer is. I see there are 2 possible destinations available for the transfer call tool: 1.

number

2.

sip

For the

sip

option it seems like I can easily provide SIP headers, however it is not clear to me if it's possible with

number

. Is it simply an issue with PSTN? Do you have any recommendations for the overall problem? I assume the solution is to set up a SIP trunk for transfers instead?

Based on the documentation, passing SIP headers (like a customer ID) is only clearly supported when transferring to a

sip

destination, not a

number

(PSTN) destination. Setting up a SIP trunk is generally recommended for this use case. To confirm: Is your call center able to receive calls via SIP, or does it only accept standard PSTN numbers? This will help determine if SIP headers can be used for your transfers. Source: - [Vapi SIP Trunk Documentation](https://docs.vapi.ai/advanced/sip/sip-trunk)

@User We're trying to figure out how much development will be required. The simplest would be PSTN numbers, but we need a proper solution where we can pass context to the agents. If we need to set up SIP, we'll do that, although it will be more development work.

I’ve run into this before with PSTN you’re pretty limited, since custom headers don’t make it through the network, so you won’t be able to pass the customer ID along that way. SIP is usually the way to solve this because you can include headers or metadata that your call center system can read as soon as the agent picks up. It doesn’t always mean a huge build, sometimes it’s just about setting up a SIP trunk and mapping the data correctly. Which call center platform are you using? That’ll help me point you to the simplest setup. @flyingemolga

We're currently using Twilio to make the Vapi calls. Internally, our call center uses some other providers. From conversations I've had with our telephony guys we would be using Asterisk for SIP. I'm investigating Twilio's Elastic SIP. Do you have any recommendations?

Since PSTN can’t pass custom headers, SIP is the way to go — Twilio Elastic SIP to Asterisk will let your agents see the customer ID instantly. I’ve helped teams set up this kind of transfer with proper header mapping. We can handle this directly and discuss the setup privately so you know exactly what’s involved.

Thanks. I've sent on documentation for Twilio's Elastic SIP to our telephony guys in the meantime. Largely we needed some direction and it seems like this is the way to go. I'll catch up with you next week once I've heard back from the Telephony guys. Thanks for the quick feedback 👍

Great to hear! Once your telephony team weighs in, I’d be happy to help make the Elastic SIP setup seamless. @flyingemolga

Great, I'll definitely pull you in 👍

Hey @flyingemolga how is it going?