Yes, you may need cell phone signal booster to use apps like whatsapp or viber. Thing is that these all applications works on data services and which directly relies on cell phone signal strength. Low signal problem has been increasing daily instead of many attempts by service providers to solve it, reason is that there is not too many places remaining to install new cell phone towers and the increment in usage of smart phones.
The low signal problem causes the low speed of data services which results in poor functioning of apps like whatsapp or viber. Whatsapp has recently introduced Voice Calling facility and Viber has it from long time, my point is that for clear voice calling through data services, you will require a proper signal coverage and if you are not in area with such signal strength then you may face many problems using such services.
Question arises is that why only cell phone signal booster can help?
Answer is simple and that is the service providers all over country are unable to solve this problem from there end and there is no other better option then cell phone signal amplifier. A cell phone signal booster use to boost low signal to full strength and spreads it all over needed area. The increase in signal strength automatically adjust the data speed to normal (as per your plan).