Finally i solved this problem regarding to "mobile
networks available" cant detect signal, cant send/text
messages issue, simply dial this number in your phone
Step 1.After phone power on dial *#*#3646633#*#* .
Step 2.A menu will appear on screen , then select GPRS.
Step 3.to change imei 1 select 1ST PDP and input new
imei and select ok.
Step 4.After this action in order first select "Attached" ,
second select "Deattached" and all will be done.
P.S. if need to change second imei just follow the same
steps, only the diffrence is on step 4 select 2ND PDP and
thats it.
I hope it helps..it works on my phone galaxy s4