I ported my Cingular/AT&T number to Google Voice many years ago and six months ago ported it to Ting. There are a number of companies who consistently fail to deliver texts to that number. The most notable (for me) is Amazon. Yet dozens of other companies (Facebook, Google, etc.) are able to use it just fine.
I don't have the slightest idea on how to fix it. The only reason it worries me a bit is that I might miss texts from other people.
Switched to an iPhone so I could use "Find Friends" with spouse and kids.
The only problem I recall is that the initial text verification doesn't work - I had to do the phone call option - but this is a problem with gvoice, not WhatsApp: google blocks pretty much all server-triggered texts.