i’ve used " " instead of blank string, and it still did not work.
i also buy other sim of Vinaphone, it require APN=“m3-world”, both username & password = “mms”, and it work well now.
so maybe there a problem with my Viettel sim, i will test it more.
thank you awneil 
Oh, well. 
It’s a long shot, but have you tried the combinations of making one an empty string, and the other a single space…? 
That’s progress! 
At least it tells you that:
- Your Fastrack can work;
- Your application can work.
I think most likely a configuration problem or some other compatibility issue with the Viettel service…
Do you have a proper technical support contact in Viettel ?
If not, have you tried asking your local Wavecom Distributor? They should be familiar with the local services available in their area…
yes, i’ve already did that.
ok, i will reply later, when i find where is the problem