This question is an exact duplicate of:
I have a business sim card with an Android phone. Now different employees have to make calls, we all work from home. Right now we switch the sim to the person who is working. I would like them to be able to log on to the phone over data/wifi and make the calls. Does anybody know a location
I have been looking for this a few weeks but cannot find a solution except GoIP + a PBX. All solutions I found for this are a bit too complicated for the time I want to invest in this.