This could indicate that the network operator you are using might have changed/reloaded custom firmware onto the device.
To resolve this you would need to contact them to confirm this.
One option you have to test with would be to download a softphone such as Zoiper on your smartphone.
Connect the phone to the router to use WiFi only, then see if you are able to connect.
If this is successful, then an ATA adapter will also work. It's a good way of testing before spending any money on hardware.
If anyone has resolved this somehow, please advise. I've put Vodacom Sim in place of Cell C, so not sure if they also a 'probem'. There seem to be reports of people using B315s-22 with an analogue phone, even the Huawei manual advises to connect a phone to the RJ11 for this model
....would be great to resolve...not a bad router....
The issue with getting a router from an ISP is they often load it with custom firmware to bar specific services. Case and point with Cell C dropping native support for VoIP using the internal ATA functionality.
You could try and flash it with vanilla Huawei firmware but this could be risky, doing it incorrectly/using the incorrect firmware could lead to bricking it. I have done it before and it required you take the unit apart (you will lose your warranty) and use a pin as a jumper to boot into the flashing mode. It's a nightmare and it requires you use Huawei flashing tools which are in Chinese....
At this point, the best option really is using a VoIP phone or you could try to find a cheap ATA adapter if you want to use your old analogue phones. I had a quick look around and it seems that they go for around R700, you could contact Miro and ask them if they would deal with out an account. At that price point, you might as well just go for a cheap Yealink.