Cannot send SMS to one specific contact!!!

[Tremblay]

My friend cannot send me SMS messages. It keeps giving an error, saying "Cannot send". Meanwhile, she's receiving everything Ive sent her. I got her to delete all our messages & delete the contact then reenter the contact. It fixed the problem for about 10 messages, then started again. There was a several hour gap before we realised it wasnt working again. She's got some Xperia phone, she just recently bought it. Gonna try and find the exact model. What else can I try?

She's also been sending and receiving SMS messages to everyone else perfectly.

[Bouvrie]

Alternatively, I suspect some other (Social Media) app may be syncing data, overwriting your phone number in the Contacts with some data from your (Google+?) profile?

Does she have some applications installed that may have write access to her contacts?

[boy_pulbos]

I have the same error....but I found out the solution

My wife is using Sony Xperia and I am using iPhone 5s.

She can sms me but I can't.  I simply disabled iMessage from my iphone 5s and I am able to send sms.