Well, where to start.
The house does have to be cleaned every so often, just as it does before you retired. Possibly more often as it doesn't sit empty and clean for most of the day. That doesn't take very long unless you are OCD and want everything absolutely spotless at all times.
Outside is the garden, where weeds are appearing hourly and growing 24/7 so you could get some exercise and fresh air and at the same time get it ready to sit out in the sun with a cuppa or a glass or two and a good book.
Talkng of books, you can borrow electronic copies of books from the library, downloaded onto the same device you used to post the question at the head of this thread, and if you have/buy a Kindle you can also buy them online from Amazon at about 99p each, or even free if you enjoy the ones available.
You could learn a new craft, or do some more of one you know already. There are thousands of videos on Youtube with ideas to set you going and detailed demonstrations with closeups of how to do it.
Online learning need not involve studying as you remember it from school, working through dull books and memorising a lot of stuff to be examined on later. There are free online courses on absolutely everything, from Algebra (that does require a bit of concentration!) to Zumba, and the descriptions of them make it quite clear what level of knowledge is being assumed and how long you can expect to have to devote to it. Two or three hours a week are about the average, and you can start from a condition of complete ignorance or add to something you are already well-up on.
The courses usually have a lot of short videos that can be paused and rerun, short talks by experts in their fields, quizzes so that you see whether you understood and a forum to chat to others who have joined. The whole thing can be done in short bursts whenever you have time and feel like it. Examples of providers are Futurelearn and Coursera, but if you do an online search for MOOCs (Massive Open Online Courses) you will find tons. There are subjects and levels to suit everyone.
The above covers just some of the things you can do in your own home, on your own.
Go out of your garden gate and find somewhere to volunteer, where you can use whatever you did while working, to help others in need or in distress or who are unable to do things for themselves. You could raise money for a local or national charity, improve your neighbourhood, walk a dog for someone housebound or at work, teach English to an incomer woman who is unable to meet and chat with neighbours in this new language and needs to understand new customs.
You could find part-time work doing something far removed from what you did at work, meet people you would never have bumped into normally. You could become a teaching assistant in a nursery class, stack shelves in the supermarket, serve behind the counter in a corner shop.
You could organise tea and cake for a few lonely retired people like yourself, where you could all chat once a week. That could be the only time some of them saw anyone else face-to-face - which sounds like your own situation, as you appear to have no contact with anyone except on your phone.
If there isn't anything there that gives you ideas, ask the friends you are (or would be) phoning all day. Perhaps they have solved =your problems for themselves already. If they haven't they might welcome your suggestions.