Plusnet to plusnet mobile calls going to voicemail

[#1]

I'm on a Plusnet SIM only deal my mum has Plusnet mobile, landline, internet.

Very frequently - like about 50% of the time - if I try to call her on a morning I hear silence for about 10 seconds then 'this call cannot be taken at the moment' and it goes to voicemail.

Sometimes there is no slience it goes directly to voicemail.

She insists the phone is switched on and has a signal.

What could be the problem?

Thanks.

[#2]

Hi there Paul, 

Sorry for us taking a long while to reply, we're really busy at the moment. It would be worth us checking for any issues in your area. Could you send me a PM with your mobile number and your full name please?