Hi all, long time lurker, first time poster.  Thanks for all the useful and informative comments in these forums.

I have an odd issue with my partner's mobile phone, where she's stopped receiving texts from 3 or 4 (possibly up to 6) people in her contacts list.  She can send texts to these people, but she hasn't received any texts from them since approximately the 19th of November.

All of these users are with Spark. 

My partner is with Vodafone.  She's called Vodafone support and they claim the issue's not with them, and report that they can see texts coming in from the affected people until the 19th, and then nothing more in their system after that date.

One of her affected friends has called Spark support and they claim the issue's not with them, and suggested there may be a problem with my partner's phone. 

My partner has an iPhone 5.  I've swapped her SIM into a reset and fresh iPhone 6, and we still have the same problem so we can rule the device out.

Here's the rest of the facts we know:

1.  All the users we're missing texts from are with Spark
2.  Apple iMessage is unaffected, ie: we still get iMessages from people on Spark
3.  We can still send texts to Spark customers
4.  The users we're not getting texts from don't receive any error or 'not delivered' messages on their devices.  It just looks like the text was sent.
5.  Neither my partner nor her affected friends have changed their devices - ie: we haven't got that weird 'Android to Apple / SMS to iMessage' thing going on.

We're at a bit of a loss here, and not sure how to proceed.  Any help or advice would be greatly appreciated.  I'm more than happy to answer any questions.

Thanks for your time, Craig