Sorry, cannot help any more. It seems to be something with your mobile. All the schematics is OK. About PDU I have no idea and to be honest, I don't want to get!!!

I always used phones with text ability and in control applications Wavecom and Telit modules.

I think that you have timing problem since the Hserin/Hserout is the best command on the PBP repertore. Of course a string handling needs to be improoved, but that's another thread story.

Ioannis