[Best practices for Grandstream GXP phones on the hosted network (getting intermittent 403 forbidden errors)]

I didn't have any issues with BLF, but it doesn't surprise me. Each new firmware release had one new feature working and several broke. Therefore, I fully understand how the $99 price tag including Bluetooth, color screen, and etc. will keep you in the, "I want to make it work" category.    

[Bridging two or more Kazoo/ Monster accounts]

I have several clients asking we bridge their offices together.

My preference is to keep their offices on separate accounts as to offer them independently the flexibility they need without complicated combined callflows.

The though is to have a way to dial directly to another account without assigning a DID. Preferably a device that can be configured with: <account>/<extension>.

Any thoughts on how this can be done?  

[Best practices for Grandstream GXP phones on the hosted network (getting intermittent 403 forbidden errors)]

These IP Phones seem like a great value for the money, but after much testing I've given up. The best I can remember is don't use an outbound proxy and if you do, use an IP address (i.e. 8.36.70.3:7000) instead of a domain name. For some reason resolving the DNS occasionally results in a 403 Forbidden error.

[After Hours Forward to Call Center]

As a technical note on landline devices, consider increasing your ring timeout to 50 seconds. It seems to be the sweet spot for many answering services and especially iphones--of course mileage will vary. Additionally, many after hours services can recognize the callerID and route the call accordingly. Therefore, check/ uncheck the "Keep Caller ID" as required by the answering service.     

[Call Recording Inbound & Outbound]

I believe you can accomplish outbound call recording by using advanced callflows. Create a "no_match" switch case (if it doesn't already exist) and set call recording prior to Global Carrier. I'm not 100% sure, but I remember doing this and it worked.

[Different ring for Internal vs external inbound calls]

From the Backend...to configure the "Alert-Info" header.
Under Device | Options
Internal Ringtones = <http://localhost>;info=Internal
External Ringtones = <http://localhost>;info=External

I'm not that familiar with Polycom, but this works for most phone types.