You may be headed in the right direction... DIY VoIP. Knowing your inbound and outbound monthly call volume, number of DIDs, and number of e911-enabled DIDs/locations might help choose a SIP provider. I would not use GV for your business as it has 'free' limitations that can upset business use.
Have you checked out SIP providers
VoIP.ms. Callcentric, and Anveo? If you had gone with
VoIP.ms at home, you'd probably see a direction forward now for your business.
Using a SIP provider like one of those above will allow you to manage your 'virtual phone system' in their cloud. Then use end point terminals/user agents to suit the need... IVRs/automated attendants, softphones on mobiles, atas with corded/cordless analog phones/fax, IP phones, answering services...
>>1) Take control of those phone lines so I can receive those calls directly and have an automated system in reserve.
Check.
>>2) Let my business partner be able to answer those calls as well, we both work remotely in separate houses.
You and your business partner can SIP register your respective endpoints to
VoIP.ms subaccounts. Then they can have extension numbers for free internal calling/call management, dedicated DIDs if required, and location specific e911 service... if he calls 911 from his office, the 911 dispatcher gets his location address, and vice-versa. And there are many call routing options you can configure in the cloud service... group ringing, call hunting, IVR, time of day restrictions, etc.
>>3) Not lose any of my existing Google Voice landline functionality, if possible. It's 4 phone lines. My friend told me about shared call appearance, but this confuses me.
To be determined. Edit: Know that your friend who sells big phone systems will naturally conjure up his usual list of sales points and system considerations, competition unseen. Learn from him but know that missions like yours are co-opting his bread and butter.
>>4) Have an app so we can receive business calls on our cell phones, or at least voicemails.
A softphone extension on your mobile may suffice, subject to the idiosyncrasies of mobile devices/connections. You can configure call routing in the cloud to include failover to voicemail. VoIP.ms vm can be sent to e-mail as .mp3 or .wav (lossless) attachments.
>>5) Possibly bring over my ringcentral efax under the same umbrella
To be determined. You can likely integrate your preferred fax solution with your preferred SIP provider. FAX/SMS/MMS functionality over SIP varies with service and equipment, but tends to lag that same functionality over POTS/cellular.
>>6) Have a virtual extension for a sales rep who also works remotely
Check. VoIP.ms offers 'unlimited' subaccounts/extensions at no additional charge. VoIP.ms offers many and most of their features at no additional charge, which greatly simplifies your consideration and use of their extensive offering. I have extensions in 4 locations, not counting the mobile softphones that travel daily.
>>Caller ID
VoIP.ms permits you to spoof the outbound CallerID sent from any extension... so set whatever CID you want, or require such as an e911-enabled DID for that extension's offsite location.
>>IVR
You can build an IVR/automated attendant in the cloud to process your inbound calls to suit your needs, time of day, virtual phone system extent, etc. (You can use vm and .wav attachments to easily prototype recordings for setting up your IVR.)
You can get an account, add a small minimum deposit ~$25, and build your virtual phone system in the
VoIP.ms cloud. Use a temp DID to get it working and when ready, port in or forward your DID(s) to it. If you like it, proceed, or shift direction a bit until satisfied. VoIP.ms has a good learning environment; if you do move on, you'll be more prepared. I never saw the need to move on although a business application may want to have backup service in the mix.
Finally, you might consider posting on the VoIP Tech Chat forum to reach more business phone system advice.
https://www.dslreports.com/forum/voipOE