Viber is asking for access code but I already have account! I have a new phone with old sim but it will not let me use viber. It says I need access code and then never sends one. How can I sign in using my viber account?
I have had similar issues with Viber on many Android devices, all running multiple versions. I've found that first clear your data, then using your entire number e.g. +61 0403233142 has seemed go help. Logic? There appears none. 

Whilst I do not have a "fix" nor the cause for this issue, there are many sites that offer their opinion. I do believe at one stage there is a manual activation method in which you're able to call a service to retrieve a code- I will try to hunt this down for you. 

Another method you could possibly try is to setup Viber from a PC client and send the activation code- it may be an issue with your phone sending the request. I will also add in some 'handy links'; hope all goes well!

Please look into this page for assistance: http://www.viberfaq.com/i-did-not-get-my-access-code-what-can-i-do/ 

The theory behind this support may also apply: http://support.viber.com/customer/portal/articles/1251053-solving-access-code-issues-iphone-

Worst comes to worse, look here and possibly try the desktop client: http://support.viber.com/customer/portal/topics/646085-most-popular?b_id=3838