I used to have my Telo behind the router, but had problems. Since my router is very good, I used the following instructions.
Some things you could try:
Access your Ooma setup and set both the Downstream and Upstream Quality of service settings to zero, update and test it with speedtest and make phone calls. Make sure you reboot your modem, router and Ooma device each time you make a change.
If that doesn't work, Test, connecting your Ooma device directly behind your Modem. Set the Up Stream and downstream Quality of service settings as I posted previously. Also while you are there, turn off Mac Address spoofing per my previous post. Make sure you reboot your modem and Ooma device each time you make a change.
Next if that doesn't work and you rent your modem, exchange your modem for another one and test.
If you own your modem make sure your modem's firmware is up to date. If that doesn't work and if you own your modem, see if you can borrow someone else's modem for testing, or purchase a new one. If it doesn't work in most cases you can return it. Check first before purchasing.
Test with http://speedtest.phonepower.com/
and check for packet loss and upload speed each time you do a test.
Report back to let us know how you are doing.