Husbands PAYG number used when his phone off

[pam]

My husband has an O2 PAYG iPhone (my old 6s)-I'm on O2 tarrif. We had cause today to ring O2 CS because of a data message appearing in his phone and were told by the CS that his phone was used to make a call to a number we didn't recognised and which he hadn't rang. We were also told that on 17/12-last Monday- he was using data being online, but his phone was switched off and left at home as we were the our gym and he never takes his phone there. On another occassion his number had been used online again at 8.55am but, being retired we were still in bed asleep with his phone agaibb switched off. 

Mu husband isn't into technology so it takes him all his time to use his phone and only used Safari at hone with our router, his date switched off. He'd receieved a message on his phone this tea time telling him he was over his data allowance and charged £1 when his phone was again switched off-hence the phone call to o2.

I am techonology lit so sort his phone out when I pass it over and make sure it's to his own requirements, which the uses are very little. 

According to the CS guy all this data about my husbands supposed data usage was on his screen, but we said it's all wrong.

Has anyone else had this problem at all? i suggest he changes his number but he doesn't want to. 

Can anyone help at all please? many thanks

[MI5 · Accepted solution]

If data is only required when at home on wifi, mobile data should be turned off completely on the phone and also check that wifi assist is turned off in mobile data settings. 

Settings >Mobile data >scroll down to wifi assist and check slider is to the left and not green.

[EmilieT]

Hi @pam, there's some good advice above, did you get a chance to give it a go yet? Please do let us know how you're getting on with this