Calls cannot be placed in Conference

The first thing to check in this situation is if the agent is typing in the right format for the number. If they are using stored numbers, check-in admin >Manage Numbers > transfer numbers page if the phone number format is correct.

Usually in India, 10 digit number is good enough for mobiles. Also, the mobile number entered with +91 will go through.

For the USA, 10 digit number is also good enough.

Sometimes it depends on the particular carrier also, but usually, the E164 format should work most of the time (Eg: +919734243751,+14089803211 )

If the number format is correct, then check if there are enough ports ( Channel capacity ) to connect the 3rd party. Sometimes your trunk would be at full capacity and hence 3rd party transfers fail.

Last resort: Raise a ticket with support and they might need to take a remote session and check whether facing any network challenges

If no network issue is found you need to capture the console and HAR logs and raise the issue to the product team.