Problem receiving sms from 1 person

Hi

Ive got an intermittent problem receiving sms texts from a friend who
uses pay you go on "3" network. I use the Motorola L6 phone on virgin
mobile pay as you go.

I seem to be able to send to them ok all the time but intermittently I'm
not receiving what they send. When this happens it says on their phone
the text has been sent ok but I never receive it. Then the problem seems
to vanish only to reappear few days later.

Neither of us seem to have any other problems sending/receiving texts
with anyone else.

This has got me puzzled has anyone heard of this happening before?

Thanks

Loz