This article helps with: Can I enter a competition with my landline phone?
We can't accept entries via a landline as that number isn't assigned to a single person like a mobile number is.
If we allowed landline entries we would have people using a shared house phone or office phone or even a public phone box. If that landline number was drawn out in the competition and we called it then we wouldn't know if the person who answered the phone was the person who made that winning entry.