Power problem with MT8870

I am making an application in which I am using a wavecom GSM modem(2403A module), AT89s52,MT8870 decoding cicuit and a 12V (7.5 Ah) battery.When I connect the whole circuit according to my application and powered it with battery , the MT8870 is not detecting any DTMF tone(or sometimes it detects ,the frequency of detecting is 2 out of 10). But when i connect separate power supply (other battery or power adapter 12V) to MT8870 then it detects the DTMF tone always. I am using 7805 voltage regulator for both microcontroller and MT8870.In the first case when the whole circuit is connected to same battery the voltage is 4.8V across MT8870 pins but it does not detect tones. But at different power supply it detects tones even after it has a voltage of 3.6 V at its pin. I think i am making some conceptual mistake but not able to find the solution.Maybe the single battery is not able to supply the power to whole circuit because the modem draw 1 AMP during calling. But I think the current requirements of MT8870 is not that much and besides it the during all operation the microcontroller works fines.Please help me it totally stuck in this problem. I want to drive whole circuit using one battery only.:sad:

4.8Volts on power pins means your regulator (7805) not work properly or your battery cannot deliver enough current. From your description the problem looks to be the battery. Measure the voltage at the input of 7805; if the measured value is different by nominal battery voltage than you need another battery.