iPhone 6 won't receive texts?

A friend of mine has an iPhone 6 with a contract that gives her 200mins phone calls, free texts and 500Mb data.
The problem is people cannot send her text messages to her phone number, they just don't go, they have to send emails.
She is with Vodafone and all they say is turn off, remove sim, replace and turn back on, but it doesn't solve the problem.
Any ideas please?

Are the senders using iPhones as well or Android phones? If iPhones, then it isn't regular text message but rather "Messages" that go over Apple's network. If the originating phone is Android, then it is always sent as a traditional SMS. iPhones only fallback to SMS i the native Messages aren't getting through.

What errors, if any, do the senders get? Can the iPhone with the issue send both Messages and SMS without any problems?

Thanks for that. I'll get back to you.

Just a quick note here.... What Ashwin mentioned should be resolved with iOS 11 when messages sent from an Android to an iPhone will be treated the same as an iMessage.

From my friend having asked the questions. I'll ask again what OS she's using, updates.

Mostly I get calls from iphones to my iphone CC they get not delivered message , my phone wont text to Android phones or iphones through number only gmail. My settings simply will not pick up my phone number as a address for messages. I think I have now exhausted every option, the last being shut down phone, wait take out sim wait put back sim wait turn on.

Reply from my friend, her iPhone is running 10.3.3, same as mine,. Is there an 11.0.0 out now??

"I also have 10.3.3 running CC, I really have tried everything possible. It a shame really its a nice phone but I've tried online chat several times, they always assure you they will solve the problem, I've tried emails, telephone calls to Apple and Vodafone, each tell you its the other ones fault, been in Vodafone shop, totally useless, Apple shop sadly the same. I've turned off, shut down removed sim card several times and put back. I am getting fed up with mobile phones altogether, but thanks for your kind support CC"

Where to go from here?