@coddiwomple
She's elderly and might have a routine that works for her, and be very reluctant to change it at such a short notice.
Massively BU to think that you (anyone, not specially you OP) are so important that everyone has to drop their plans and change their days to accommodate you when you feel like popping in, because it's convenient for ^you". You might have a leisurely life, and nothing better to do, but other people are busy, have plans, have a messy house they don't want to show, there are plenty of reasons.
The worst people are the ones who think working from home means being free for a coffee and a chat all day.
All of this ^
Especially this bit...
Massively BU to think that you (anyone, not specially you OP) are so important that everyone has to drop their plans and change their days to accommodate you when you feel like popping in, because it's convenient for "you"
And yeah, it is so annoying that people think you are free all the time because you do not have paid employment away from the house . And so are people who think you are available to look after your kids when it suits them too - just because you work from home.
Another thing that is annoying, is when someone is running a catalogue and they have multiple deliveries every day when they're not in, and they expect people to take them in and keep them til they can be bothered to pick them up. A woman in my road was doing this a while back (she has moved now,) and a few of us had to put a sign up on our front doors, saying 'we do not take in parcels for neighbours, so do not knock for this, and ask us to do so...'
I know someone who is a stay at home mum/housewife who is in her mid 50's. Her kids have left home, and she does not work (took early retirement at 50.) She had such a barrage of people expecting her to babysit, run errands, take them to appointments, take multiple packages in, give them lifts, etc etc, that after 2 years of it, she told them all she had got a part time job with random unpredictable hours, and would no longer be available for anything.
Just because she is without kids at home, and does not have a job, people assume she is just sitting there waiting for them to call and let her know what services they need from her! Only me and her husband and her daughter know she doesn't really have a job!
@LemonadeWithACherry
Thank you for what you said Ivy.
I hate unannounced visitors and do not answer the phone if I don't know the number. Both of those things cause me unnecessary stress that I can do without as I have MS and anxiety.
However none of that applies to my children, grandchildren or especially a new great grandchild I've never met! And it sounds like if she did have a genuine reason she could have said it more graciously. Having ironing to do is a shitty excuse to give, that a person couldn't give without realising how rude it sounds in my opinion.
You're welcome. 
Yeah it can be very stressful and upsetting for some people who suffer anxiety, and to mock and berate and take the piss because some people don't like unexpected visitors, is just rude.
@sloesloequickquickgin
Not forgetting that some won't answer their phone unless they are expecting a call so the visitors couldn't call in advance to arrange a visit anyway
Don't think I know anyone who doesn't answer the phone! 
Only someone who said they won't answer if they don't recognise the number. And why should they???
As I said, mocking people who think differently to you, and may suffer anxiety, is just plain rude.
Maybe your attitude towards them is very apparent, and this is why they don't like you visiting them.