I often use Skype to send SMS to mobiles. It has always been very reliable but recently I have found a problem sending to a friend who is on the True system. The message just refuses to go through although if I send via a phone it's OK. Anyone else found this? Is it a network thing or maybe down to the handset?

You should probably check with Skype. Were you ever able to successfully originate SMSes to TrueMove subscribers? Is the destination a TrueMove or TrueMove H number? In theory Skype would have to have an agreement with TrueMove, and access to their SMS gateway. TrueMove is a 2nd tier provider here so maybe they don't have any agreements with Skype?

I have only been able to use the Skype SMS service to the AIS network. It has never worked with DTAC or True