Why isn't there something that turns a rooted Android phone into a VOIP <-> Cell network gateway? Something that allows incoming calls to be forwarded to SIP, and vice versa?
Maybe because nobody had a strong enough use case to feel compelled to implement it. There are very cheap GSM to SIP gateways, that are way less expensive than a mobile phone.
Do you have any links for those? I can't imagine them being much cheaper than a free used smartphone that many people have lying around or the price of a used phone from a pawnshop or ebay -- $50.
Not possible with most devices. I wanted something similar, no result. Seemed like a hardware restriction that phone calls go to either the speaker or Bluetooth.