I notice some complaints about battery life.
I don't use an E-Caller at the moment but here's some food for thought (I'd suggest checking the owners manual for verification). The unit should have a desired "type" battery, Alkaline, NiMh or Lithium.
Alkaline- One time use, power output depends on manufacture. Dies very quickly in cold/freezing weather.
NiMh- Verry similar to the alkaline except rechargeable (I regularly use duracell NiMh rechargeables for my electronic needs. I bought a pack of like 30+ batterys and a charger and they have been working top notch for the past year. Just make sure to fully charge and discharge them when using them.)
Lithium- Long lasting and expensive. It is not rechargeable but the major benefit to lithium is that freezing weather doesn't affect the life/power output at all.
I suggest trying different batterys and noting any difference.