Phone loses connection with base?

I have 3 V Tech phones Model i5808. Lately each phone loses connection after 4-5 minutes of talking. I'm suspecting it not to be a battery problem since all three phones are having the same problem at the same time and they are in different locations throughout my house. I get messages such as "cannot locate base" and "base not found". These are cordless phones and my carrier is Vonage.\r\nPlease advise.
Thanks