longer ring time before switching to Voicemail.

Hello, how can I set a longer call ringing time on my Samsung A15 5G before it switches to Voicemail.
At the moment it's about 15 seconds, which isn't long enough for me to answer the phone at times.
Thanks for any help.
Jon

Did you get any joy?
I think we have to talk to Lebara to get a result. My phone only rings for less than 10 seconds.

Hello Max, Well, I googled 'how to .....' for my specific make of phone and it came up with the correct sequence of digits to dial which worked.
I was able to increase the ringing time easily.
But my phone didn't divert to voicemail at the end of ringtone.
I then found my phone was directing the calls to a different number - not my usual voicemail number.
Put that right, and everything now works properly.