when you have your delivery status on and you send a txt and the report Reads> Delivery status...Unsuccessful ...and it is oonly happening with one number what does this mean? in a bad service area ,phone off anything like that and theres is a prepaid..
The only thing one your end that I can think of that could cause messages to ONE number not getting through is if the number isn't typed correctly. Make sure you enter the area code for all text messages, even local ones.
Otherwise there is a myriad of reasons as to why it hasn't gone through on the end of the intended recepient. The phone might be off, not enough memory, out of coverage zone, broken phone, and so on.
I'd recommend having your contact call their tech support (are they with Koodo? they should be) to figure it out.