FRITZ!Box is not only a great router for home fibre installations, but it also has a built-in SIP server that works with 2talk and Grandstream IP Phones!

Products Used:
FRITZBox 7490 Gigabit 802.11ac ADSL / Fibre Wireless Router
Grandstream GXP2160 6 Line IP Phone

Step 1:
Login to your FRITZ!Box and navigate to Wizards > Manage Telephone Numbers.

Step 2:
Select Add Telephone Number followed by IP-based line.

Step 3:
Enter the following settings to register your 2talk number on your FRITZ!Box:
Telephony Provider: Other provider
Telephone number for registration: 2talk_phone_number
Internal telephone number in the FRITZ!Box: Any 2 or 3 digit number should be fine
User name: 2talk_phone_number
Password: 2talk_password
Proxy server:
DTMF transmission: Automatically
Insert area code for outgoing calls: No

Step 4:
Save Settings and verify your phone number is registered:

Step 5:
Now our 2talk phone number is registered to our FRITZ!Box we can setup an IP Phone (an old analog phone will also work but is not covered here) for inbound and outbound calling.
Navigate to Wizards > Manage Telephony Devices and select Configure New Device

Step 6:
We want to select a Telephone Connected to the FRITZ!Box, and for the type of telephone select LAN/WAN (IP telephone).

Step 7:
Now we need to configure a SIP User Extension and Password for the Grandstream IP Phone to connect with:

Step 8:
Now that we have created a SIP extension for the Grandstream IP Phone to connect to we are now ready to provision the Phone with these settings. Start by navigating to Network > Devices and Users to locate the IP Phone on your network, once you have located it you can simply click the IP Address to connect to the Phones web interface.

Step 9:
Once logged into the phone, navigate to Accounts > Account 1 > General Settings and fill in the following:
Account Name: Any Name is fine
SIP Server:
SIP User ID: 620
Authentication ID: 620
Authentication Password: 620
Save and Apply Settings

Step 10:
Try making an inbound call to your 2talk number to verify it works, also attempt an outbound call from the Grandstream IP Phone.