Poor signal on I Phone

Since yours is an iPhone, you can download FaceTime. There's something called WhatsApp, which you can download and use , provided the rest of your contacts also use whatsapp. These are free to use.
I use SKY wifi. No trouble at all.

I use Plusnet mobile with an iPhone without any issues. Have you contacted Plusnet to check if there is a problem?

If it’s not giving you the service you signed up for keep on at them I m with EE and have no problem with coverage on my iPhone

I've contact Plusnet three times, they sent me another sim saying this might help, couldn't understand how changing the sim could alter a signal.
Another person said they couldn't do anything else about the signal strength.
We do have a problem with signal where I live and EE manage to sort this out for me when I was with them.
Since Plusnet use EE I assumed I would be alright.
I use Facetime and Whatsap and have the same problems with the signal faltering.
That's why I wondered if using Skype on our home WiFi for making phone calls would work.
I wasn't sure if you can download Skype to the IPhone and use it for making calls.
I've made calls on Skype which is installed on my laptop and it worked OK.
I changed the setting on my phone to WiFi calling and assumed making a call would go via the Wifi!! it didn't work so what is the Wifi calling for if not that!! I don't understand.

Different companies are better for different areas unless you live in my sons house where no one get a signal! Our Sky engineer recently told us to change to BT as it is better for WiFi in this area! I am on 02 for my mobile and generally get good service

Different networks have varying signal strengths according to your location, Vodafone is good for me, EE was useless, it’s nothing to do with the phone.
Some, maybe even all networks publish signal strength maps, choose a network with the best strength in your area.

It might be your phone.
iPhones are having problems lately (so I read)
I have a Plusnet SIM and get a good signal but there have been odd times when the call has cut out for no apparent reason.
I too use Wi-Fi for making calls and that makes no difference.
I have to ring back and apologise in case they think I've cut the call but can never speak to the same call handler, therefore I have to start the conversation all over again.

Are you near to visit an Apple store to make an appointment? Do you update the phone when there is an IOS update and go through your settings to check they are all as you need?

I am on EE with an iPhone, and I have been having a lot of problems for maybe 6 weeks now. Weak signals in areas where I used to get a good one, erratic email service, even WhatsApp sometimes 'hangs', so there is a delay between texts being sent and received.

A friend of mine has the same phone, but lives about 15 miles from me, (so usually gets a signal from a different mast) is having the same issues. We used to work together, and for many years changed our phones within a month or so of one another, and noticed that this sort of thing always seems to happen when a new phone comes out .

My iPhone has been out of contract for 9 months now, and I keep getting adverts for a new one - funny, that.