The most important factor is how the iphone and the android phones roam and perform. If iphones and android phones performed good with sip clients, handset manufacturers would be out of business. It is not impossible to get them to be reliable VOIP clients, but it takes alot of work. In addition, every android handset and every version of the iphone does not roam the same, so it would involve trial and error. I would work with the SIP client manufacturer to get information on how to deploy your wifi. If they have an existing customer that you can visit, as well, that would be good.
Like it said, it is possible, but it can be difficult.