Are the compatibility issues between modems and kernels? Can i try any modem with my kernel?

someone?

Firstly you need to eliminate, if its a carrier issue or phone. you can try a different sim card. or try your sim in another phone...

Also what is your signal in -Db (you can see this in phone status).
Another thing you appear to be on a very old ROM, try flashing to the latest official Samsung Rom.

if all this fails then it looks like hardware issue. You could try using headphones to eliminate if the speaker and mic issue.

just think logically...I'm sure it can be fixed.

Ok...first, it's not a carrier issue because my girlfried is using the same carrier and has no problems. Second the signal strength is "-75 dBm 19 asu", but like I said, the signal appears to be full when the phone conversation is stating to have problems and I'm not walking when I'm on the phone ( the signal can not be lost by changing the place from where the phone is used )...I have 3 or 4 bars all the time...so I don't think it's a hardware problem because if it were I shouldn't be able to have good signal at all, but I do. That problem doesn't always appear.
I know that I should change the modem but I don't want to decrease the battery life. So, my questions are:
1. If I change the modem will the battery life be affected? If yes, what is the best modem for battery lifetime? If no, it is deppending on kernel:
2. Are there compatibility issues between modems and kernels?

Thanks!

the modem changes depending on the area where you are and the operators.

In italy i use i9000xxjvu and this modem work very well and consumes very little battery

Yes different modem firmware will affect the battery performance.. some are better than others..... why don't you take a nandroid backup of your current rom...and test a complete new rom with latest modem firmware..just install a stock official latest rom for test purpose. If it still bad , you can easily restore from the backup But I think if your signal - 75 DB (which is like excellent) and your having problems hearing ' speaking ... then it looks like speaker ,mic.. use your supplied headphone to answer calls..as another test , to see how it responds

if you hve stock non rooted firmwre from your carrier or for your country it is plain stupid to play with modems. samsung knows how to fit proper modem for particular region.

parameters you presented also indicates that there is nothing wrong with modem<->bts communication, signal is more than decent.

this might be a problem with some application or some previously installed application that messed up with phone.

i would advise:
- backup of any data you will need
- backup of apps
- MAKE FACTORY RESET from recovery menu or via dialpad code
- retore APP ONLY and USER APP NOT SYSTEM APP
- do not restore app data, or retore app data one by one checking if that affects your phone

if that wont hellp, youre clear to send your phone to samsung repair center