In Australia it used to be instant write-off for business assets under $300, then they raised it to $30,000, and now it's $150,000 until the end of the year.
Albeit must be under $500M annual turnover... so if we ever strike a hit app goldmine, we could be in trouble.
Claiming second (and third and fourth...) mobile phones as a business expense for test equipment should be no problem in any country. Your main day-to-day phone might be trickier; I've yet to be challenged about claiming the entire cost and, happily, our politicians and bureaucrats have provided mountains of examples supporting my interpretation of the rules. ?
If that argument doesn't fly, then my backup argument is to cost out the personal-use component as being (cost of phone being used for business and personal use) minus (cost of phone being used for business use only) which, in these days of unlimited plans, is $0. This reasoning also works for internet service. Home phone is a little tricker, but I have a VOIP service at $4/month including DID and free landline calls within Australia (that I do on the mobile anyway) so I don't even bother trying to claim that.