If your bf was in his garage and the police had an arrest warrant they didn't do anything wrong. Police had a right to search persons physically close to your bf for officer safety. I don't know why they seized your phone unless maybe you were using it to interfere with the arrest. There could be a hundred reasons but we don't have enough facts to say. You should hire an attorney to get your phone back. Even if he doesn't succeed you'll know the reason it was taken.
As Mr. Kaman opines, there could be a number of reasons that your purse was searched and/or phone was seized by law enforcement. The phone is not "stolen" but seized without a warrant depending on the circumstances. Do not post any more facts since it is a public forum. Talk to an attorney in your area since.