Sigmatel sells mobile phones with valid IMEI codes and this was indeed verified by you.
If a SIM reports ‘Invalid SIM’, it can mean two things:
1. The SIM slot is dead.
2. The SIM is dead because either short-circuited or the cellphone operator deactivated your account

You can very easily check for problem #1 by taking out the problematic SIM and placing the working SIM in that slot. i.e. assuming SIM-1 is working in SLOT-1 and SIM-2 is not working in SLOT-2, take out SIM-2 and place SIM-1 in SLOT-2 socket. If SIM-1 (working SIM) works in both SLOT-1 & SLOT-2, you will know that both the slots on the mobile phone are working and your phone is fine.

You can diagnose if SIM-2 (non working SIM) to check for dead SIM or deactivated SIM by trying it out either in SLOT-1 or any other mobile phone.

It is also possible that SIM-2 is not supported by your mobile phone. Sigmatel phones are Dual-Band phones and some mobile phone operators on India (the new ones) operate on Tri-Band frequency.

HTH

I possess a Mobile phone set Make – SIGMATEL (Dual SIM). Since last fortnight or so, SIM-1 of my mobile handset is only functioning.On operation of sim-2 the screen shows “INVALID SIM”. But on 2211.2009 seeing an ad I sent a message to 53232 to veryfy my IMEI No. of handset from both the SIM. Through both the SIM I received message – “This belongs to BRAND SIGMATELT33iMANIFACTURERYuceller Elektronik Dis Tic Ltd Sti Source: MSAI. I solicit your kind advicein this regards.With thanks.