It is very easy to unlock the spv c550 even without any sim! it toke me just five minutes and it was free, thanks to MODACO and SPV Developers
I will quote them
Connect your spv-c550 to your PC via USB and let it syncro by ActiveSync.
The spv-c550 should be keeped connected to your PC during the entire following process (with the ActiveSync running). This should be automatic.
then, you have to Application Unlocking in order to install and use programs.
You have to install a Registry Editor in your phone. The problem is that you can't transfer any *.exe file to your phone, but you may overcome this issue by zipping it in your Pc, then transfer the zip to the phone, unzipping it there and running the exe in the spv c550.
to have the Registry Editor please use the attached file (below)
Now you can edit the following Registry entries:
HKLM\Security\Policies\Policies\00001001 = 1 (RAPI)
HKLM\Security\Policies\Policies\00001005 = 40 (Cert)
How to do:
run the regeditSTG in your spv-c550
select and click on HKEY_LOCAL_MACHINE
go to Security and click, there go to Policies and click, there go to Policies and click, then click on the left soft key Values, then select and click 00001001 and change the value data to 1, then click on the left soft key Done. Now select the 00001005 and change the value to 40, click done. Now back back back... until exit the regeditSTG. At this point you have done it.
Once this values have been changed your spv-c550 should be Application Unlock (thanks to Zone-MR at SPV-Developers)
Now it is time to finally SIM Un-Lock:
download and use the SPV-Services client.
If I remember well you should first unzip the file in your PC, copy and run the Cert.cab in your spv-c550 (you can do it thanks to the Application unlock you have done before), the run the SPVServices.exe form your PC and follow the instructions to SIM Unlock