Both the Amazon Echo and Google Home can place outbound phone calls to U.S. and Canada for free (I love free), so my first thought is they would make really great speakerphones for conference calls. But the problem is neither can enter the access codes or any other prompt responses, they can just dial the initial phone number. But all it takes is a DTMF (Dual-Tone Multi-Frequency) generator, like these apps, to mimic the tones a phone produces. It takes a little getting used to, as the smart speakers need a brief pause to recognize and pass on the tones, so press the "keys" a little longer than usual, and it should work for you too.
Google Android >> https://play.google.com/store/apps/details?id=com.bytestemplar.tonedef
Apple iOS >> https://itunes.apple.com/us/app/dtmf-pad/id312536578