Caller hears customer only on second call

Hello all,
I have a strange problem with a customer. The customer complains that every time he calls, the called person does not hear him. If he then hangs up and calls again, then it works. He has the problem not only with one call, but with several calls with different numbers. I have not asked now exactly which numbers these are. Maybe then it would have been possible to determine if they always come from a stranger’s network.

Do you guys have any idea what this could be due to? A bad device?

We are here in Germany and use T-Mobile. (not in the city)
The customer uses an iPhone 7 with the latest iOS.
I have already spoken to T-Mobile and they said it is not a network issue.

Many greetings

So many potential elements to cause the issue.

Given the customer has tried multiple different numbers (and I assume phone devices) and replicated the problem, I would suspect its something to do with the call routing from their local network to yours.

Unfortunately it would be quite easy for a telco to dismiss such a problem without specific logs demonstrating the issue (either provided to them or picked up in their monitoring systems); especially if you are one of a very small number of customers (or maybe even the only customer) having the problem.

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I had this on T-Mobile Germany with my old Huawei P20 company phone. I upgraded to an iPhone SE and the problem went away. This was with other T-Mobile numbers, as well as Festnetz (landlines). And it was that way for incoming and outgoing calls.

The problem was worse using WLAN calling (Wi-Fi calling), which I pretty much had to use, as my home office is in the cellar.

Interestingly, if the other side waited 20-30 seconds, my audio would start.

I don’t have the problem with my Samsung S20+ on T-Mobile.

And, it is never a network issue with Deutsche Telekom, even when it is a network issue.

At a guess, it might have something to do with the move to 5G, maybe they are upgrading the firmware of their towers and this is causing some issues with older phones. Maybe they aren’t 100% standards compliant and the new firmware in the towers/hubs aren’t as lenient as they used to be…

Thank you very much for the answers.

I will now try the following again with the customer.

  • He has to a contract 3 x Sim cards and one of them is still in his iPad. We try to put these in his iPhone.

  • Finally, I will reset the iPhone from him.

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