Cheapest Way to Forward Calls

Hi all
I just joined a firm where they issue you a new mobile phone and number and they do not allow you to port your number in. I was wondering what would be the cheapest way of holding on to my old number and also not having to carry 2 phones at the same time. Ideally what I want to do is to forward all calls from my personal number to the work number and keep the other phone at home. What would be the cheapest way of doing this? Expecting 5 calls a day max totalling a talk time of 15 mins. Thanks