serial communication problem

Hi, i´m trying to build a gsm alarm with my arduino and a siemens me45 phone. i´m facing some difficulties when i try to establish a serial connection between them. I´m able to send AT commands to the phone successfully so the communication from the Arduino to the phone works well. My problem is, that if i want to "listen" to the phone on its Tx pin, it simply doesn´t work. The voltage level between GND and Tx pin on the phone is 2,6V. I thought, that i can connect the arduino´s Rx pin with the phone´s Tx pin directly, but obviously that´s not correct. My point is, that i want to read incoming SMS messages through the phone´s Tx pin using them to control the alarm. Am I missing anything? Thanks in advance

It sounds like the phone's internal working voltage is something like 2.8V or 3.3V which means that: - By driving 5V into it from the Arduino you are probably damaging it - 2.6V is not high enough for the Arduino to reliably recognize a logic "high" level

Here is some information on connecting a 5V board to a 3.3V system. For a 2.8V-or-so system, a bit more work is required as the Tx signal back from the phone needs to be brought closer to 5V. Maybe something like this:

Oops! Just saw on your schematic that you already did have a voltage divider on the Tx output so you were not damaging your phone!

Thank you for your help. I thought that 2,6V will be enough for the Arduino 5V logic level. But it doesnt work. I´ve made some measurements and if i connect it directly, the voltage level is continually changing. So I ll try to bring the voltage closer, hope it ll help.