Why is my GT-15503 samsung not detecting my sim card?

i don't know if the problem is the phone the sim card slot because what ever sim card I insert nothing is detected

That's what happened to me but I have a Samsung Galaxy Ace I found out my sim card was not in the right way (Weird right) I nor my family had turned it around because I always have my phone with me. So try turning your sim card around as in around like not the other side the other way (I hope you know what I mean if not Just comment on this and tell me and same for anyone else x ) Hope this helped! :) ;)

What's your Network provider?
You may just try to check the some steps for:
1.Reinsert SIM in your mobile
2.Another SIM Insert your mobile
3.Your SIM another one mobile insert
again this trouble found..Your Samsung GT-15503 locked from any one of the GSM network.. you can get the unlock code for your mobile from online third party providers like http://www.mobileunlocksolutions.com/samsung-galaxy-5/rs1wp9/ After you get the code visit http://www.mobileunlockguide.com/samsung-galaxy-5 for free instruction on how to use that code to unlock your mobile.