International Calls being cut after a period

[Lost in Isaan]

Of late my international calls have been cut - with a musical chime - after 5 mins 14 secs, as seen on my call log.

I use AIS (Thailand) pre paid with the 009 dialing prefix on a Galaxy J7

My service provider is at a loss to explain it and logic tells me that it should not be a service provider issue, they wanting to fleece as much of my credit as possible.

I'm not using any sort of a 3rd party app to control calls but just had a major firmware upgrade which seems to co-inside with the problem.

Anyone else seen this type of problem or have a solution?

Thank you