I have a Panasonic KXTG 2740 phone with base and four extension phones. Recently I cannot dial out at all. Basically when I hit "talk" button, the read out says "talk" just like it used to but it's dialing out and there is no dial tone. I have tried a few things but nothing seems to work. At first I thought it might be the main battery and had it replaced and then charged all the phones for a day or two... still nothing. What could this be??