My mobile phone is LGP503, and I have questions about SMS and Fetion.

Because you don't use Fetion client, these are two different softwares. For example, you can't use QQ to chat with email or ordinary SMS users, although the basic function is text transmission. Fetion is bound to the mobile phone number (the so-called "seamless docking" is realized in this way), so when you send him a text message, the non-Fetion client must forward it to his mobile phone through the mobile server. Speaking of which, do you know how to solve it?