send SMS view GSM 3G Modem and VB.net Works inconsistent

I am sending SMS by using VB.net and Huawei 3G GSM Modem. it works fine if I am using GSM network A . But when I try to send SMS using Network B , it works well for the first 2 messages, eg, take 4 sec for each SMS. However 3rd SMS take 9 seconds, then 4th SMS takes 20 seconds. I some times get Error "+CMS ERROR 500" though I have credit into line , Network signal is good (99%) and phone number is valid. Investigation : - I have tried sending SMS by mobile Partner, and everything goes well. takes 3 seconds to each SMS and this is almost the same for all messages. - I have tried to use same AT commands sequence of Mobile Partner, but have same issue. - Is this related to serial port settings? - Is there a AT command needed to set modem before sending ? What could be a reason to investigate? VB.Net attached