My Phone Calls Resetting Third Party Phone

[smiffffffy]

ok this is a real weird one.
my mothers phone is a nokia 301 on vodafone network, and i am on an i-phone6s with Three network.
just last week, this issue started. when i ring my mothers phone number her phone shows that i am calling for about 5 seconds, but does not ring, so she cannot hear that i am calling.
As i continue to get ring tone at my end i wait and it eventually cuts off to NU/call cut off tone.
However now the weird bit. Her phone screen goes totally white and the phone does a full power down and restarts itself.
it does this EVERY time i call her and can be replicated.
What i have tried and each time it still happens
. checked the names directory in mums phone and removed me any duplicates of my number
. removed my number completely from her names directory
. turned off via my i-phone my number being presented to the network (unknown caller id)
. changed my mums sim to another nokia 301, the fault follows the sim.
. had the sim replaced by vodafone, fault still remains and moves with sim.
. placed another sim different number on vodafone into the nokia 301 (all ok)
. i ring another vodafone customer sim in another phone (all ok )
. reprogrammed my number into the phone.
. visited vodafone who managed to repeat everything i was complaining about and although very confused, they re-input my contact details into the first name ONLY box and NOT across the first name / surname boxes.
This did work in the shop for 4 tests of ringing into my mums phone and although unknown why /how. Today it has stopped again and upon calling in i get at my end about 30 seconds of ring tone before cutting off to nu tone when her phone restarts.
Other contacts she has who have tested dialling in get her pre-programmed answer machine message because the phone is NOT restarting when they dial in.


my wife is on three on the same phone type i-phone 6s, however her phone was working ok when used as a test call to my mums but now from today her phone on the same three network and contract as mine is now doing the same.
Could it be an i-phone update that has caused this????????
HOW CAN my phone cause her phone to do a full power down and restart just by calling is what i cannot get my head around, and even when i turn off presenting my number to the network.

I think my next steps will be
. to factory reset her phone and wipe the sim of all contacts.
. erase her from my phone contacts and try calling.

Any other things to try would be greatly accepted.

[spathiphyllum]

it sounds like the issue is with the sim card, not the iphone.. IE, the issue is at the nokias end.