Before switching to Xfinity Voice from AT&T

As a customer like yourself, who currently has the Xfinity Voice home phone service, here are the things I can answer or provide insight into:

Is it better to go with Xfinity Gateway first then buy my own gateway?

Already have internet and TV with Comcast.

I currently have my own modem and router.

I already found articles that cover turning off Hotspot WiFi and turning off Network (Bridging mode) so I can use my wireless router.

It really is a matter of personal preference to either rent the cable modem from Xfinity or to purchase your own, but if you are unsure of how long you will keep the Xfinity Voice service it might be best to start with renting the modem and if you stick with the voice service then later on purchase your own modem.  The cable modem necessary for Xfinity Voice use is different than the 'standard' cable modem because it must contain an eMta jack to plug your phone into.  There is no setup for Xfinity Voice to use these ATA devices you see that plug into an ethernet port of your router and the phone plugs into the ATA device.  For information about this please review the information located at https://www.xfinity.com/support/articles/requirements-for-equipment-used-with-phone and on that page selecting the link for mydevices will allow you to input your address and download speed to see a list of modems, which when the list is presented you will need to check the Voice/Telephone Enabled box on the left and let the information refresh before you will see a listing of modems you can purchase for use with Xfinity Voice.  Additional information can be found at https://www.xfinity.com/support/articles/list-of-approved-cable-modems

How long does it take to port a number from AT&T to Xfinity?

It has been quite a few years ago since I went with Xfinity Voice, but at that time what I was informed of is that my install date would need to be scheduled about five business days from the date of placing my order to allow time for the number to be ported over so that on install date the number would be active.  At that time I was coming from an AT&T POTS type phone.  It may be different if you are coming from an AT&T fiber phone setup.  It is best to ask the person you are speaking with when you place the order.

Can you set call forwarding to only be active if or 
when the phone service is not available, such as a power outage 
or when there is break in the outside line.

If having Xfinity Voice service during a power outage is needed then it would be best to have a battery backup/ups unit that the Xfinity cable modem and the base unit of the cordless phone system can be plugged into.  Most of the available cable modems that support Xfinity Voice service do have the ability to have a battery backup installed in them, which can be purchased separately if so desired leaving only the need to have some sort of backup for the base unit of the cordless phone. 

While it is possible through the Xfinity web interface to set up what is called ''call forwarding" it is actually a simultaneous ring where your home phone and alternate numbers all ring when incoming calls are received.  I personally do not know if this would suffice or function when a power outage happens and no battery backup to the phone and cable modem are present or if there is issues with the actual cable itself.    Perhaps someone with more knowledge or experience can chime in on this point.

Can you activate phone service via the browser instead of the app?

I do not know of any way that you alone could active the Xfinity Voice service on your account due to the number of moving parts that have to take place behind the scene with porting the number from your current provider to Xfinity, to properly provisioning the cable modem with the eMTA device in it.  Not sure what app you are referring to, but there isn't a place in the My Account app to do this.

Can you send and receive faxes?

I have one phone line active through my service and have on the very rare occasion both sent and received faxes.  I knew the time when the incoming fax would hit or when I would be sending so I just unplugged my phone from the back of the cable modem and plugged the fax system into it and then disconnected it when I was done plugging the phone back in.  If you do a lot of faxing then it might be beneficial to explore what the charge is for a second phone line and have a dedicated fax number with the fax machine connected to the second phone port on the back of the cable modem (if you desire this then you would need to be sure and acquire a cable modem that was equipped with two eMTA phone jacks in the back of it).

The other questions I can't really begin to answer as it would take someone from Xfinity to provide those answers to you.

Hope this helps out.